Add role management validation to prevent removal of the last admin

- Introduced a new error, `CannotRemoveLastAdmin`, to handle attempts to downgrade the last admin user in the system.
- Updated `RoleService` to check the number of admin users before allowing a role change that would remove the last admin.
- Enhanced unit tests to verify the new behavior, ensuring that attempts to downgrade the last admin correctly propagate the failure.
- Updated API documentation to reflect the new validation logic and its implications for role management.

backend/tests/PnvPanel.Application.Tests/Admin/Support/ApproveRoleRequestCommandHandlerTests.cs

@@ -1,4 +1,5 @@
 using NSubstitute;
+using PnvPanel.Application.Admin.Roles;
 using PnvPanel.Application.Admin.Support;
 using PnvPanel.Application.Common.Interfaces;
 using PnvPanel.Application.Common.Models;
@@ -56,7 +57,12 @@ public class ApproveRoleRequestCommandHandlerTests
             .ChangeUserRoleAsync(userId, newRoleId, Arg.Any<CancellationToken>());
         await _telegramNotifier
             .Received(1)
-            .NotifyUserAsync(userId, Arg.Any<string>(), Arg.Any<string?>(), Arg.Any<CancellationToken>());
+            .NotifyUserAsync(
+                userId,
+                Arg.Any<string>(),
+                Arg.Any<string?>(),
+                Arg.Any<CancellationToken>()
+            );
     }
 
     [Fact]
@@ -123,4 +129,72 @@ public class ApproveRoleRequestCommandHandlerTests
         Assert.False(result.IsSuccess);
         Assert.Equal(SupportErrors.NotRoleRequest, result.Error);
     }
+
+    [Fact]
+    public async Task Handle_WhenTicketIsOwnedByAdmin_StillApproves()
+    {
+        // Одобрить свою же заявку можно — единственный реальный риск (снять admin с последнего
+        // администратора) ловит RoleService.ChangeUserRoleAsync, а не этот хендлер (см. соседний тест).
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var adminId = Guid.NewGuid();
+        var roleId = Guid.NewGuid();
+        var ticket = SupportTicket.CreateRoleRequestForExistingRole(adminId, roleId);
+        dbContext.SupportTickets.Add(ticket);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        _roleService
+            .ChangeUserRoleAsync(adminId, roleId, Arg.Any<CancellationToken>())
+            .Returns(Result.Success());
+
+        var currentUser = FakeCurrentUser.Authenticated(adminId, "admin");
+        var handler = new ApproveRoleRequestCommandHandler(
+            dbContext,
+            _roleService,
+            _notifier,
+            _telegramNotifier,
+            currentUser
+        );
+
+        var result = await handler.Handle(
+            new ApproveRoleRequestCommand(ticket.Id),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(TicketStatus.Resolved, ticket.Status);
+    }
+
+    [Fact]
+    public async Task Handle_WhenRoleServiceRefusesLastAdminDowngrade_PropagatesFailure()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var adminId = Guid.NewGuid();
+        var roleId = Guid.NewGuid();
+        var ticket = SupportTicket.CreateRoleRequestForExistingRole(adminId, roleId);
+        dbContext.SupportTickets.Add(ticket);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        _roleService
+            .ChangeUserRoleAsync(adminId, roleId, Arg.Any<CancellationToken>())
+            .Returns(Result.Failure(RoleErrors.CannotRemoveLastAdmin));
+
+        var currentUser = FakeCurrentUser.Authenticated(adminId, "admin");
+        var handler = new ApproveRoleRequestCommandHandler(
+            dbContext,
+            _roleService,
+            _notifier,
+            _telegramNotifier,
+            currentUser
+        );
+
+        var result = await handler.Handle(
+            new ApproveRoleRequestCommand(ticket.Id),
+            CancellationToken.None
+        );
+
+        Assert.False(result.IsSuccess);
+        Assert.Equal(RoleErrors.CannotRemoveLastAdmin, result.Error);
+        // Тикет остаётся Open — можно повторить попытку после назначения второго админа.
+        Assert.Equal(TicketStatus.Open, ticket.Status);
+    }
 }

backend/tests/PnvPanel.Application.Tests/Admin/Support/CloseTicketCommandHandlerTests.cs

@@ -0,0 +1,68 @@
+using NSubstitute;
+using PnvPanel.Application.Admin.Support;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Tests.TestSupport;
+using PnvPanel.Domain.Support;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Admin.Support;
+
+public class CloseTicketCommandHandlerTests
+{
+    private readonly IRealtimeNotifier _notifier = Substitute.For<IRealtimeNotifier>();
+    private readonly ITelegramNotifier _telegramNotifier = Substitute.For<ITelegramNotifier>();
+
+    [Fact]
+    public async Task Handle_ClosesOpenTicket()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var userId = Guid.NewGuid();
+        var ticket = SupportTicket.CreateBugReport(userId);
+        dbContext.SupportTickets.Add(ticket);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var currentUser = FakeCurrentUser.Authenticated(Guid.NewGuid(), "admin");
+        var handler = new CloseTicketCommandHandler(
+            dbContext,
+            _notifier,
+            _telegramNotifier,
+            currentUser
+        );
+
+        var result = await handler.Handle(
+            new CloseTicketCommand(ticket.Id),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(TicketStatus.Closed, ticket.Status);
+    }
+
+    [Fact]
+    public async Task Handle_WhenTicketIsOwnedByAdmin_StillCloses()
+    {
+        // Закрыть свой же тикет можно — Close не меняет роль, риска нет (иначе одинокий админ не
+        // смог бы почистить случайно созданный тикет никогда).
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var adminId = Guid.NewGuid();
+        var ticket = SupportTicket.CreateBugReport(adminId);
+        dbContext.SupportTickets.Add(ticket);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var currentUser = FakeCurrentUser.Authenticated(adminId, "admin");
+        var handler = new CloseTicketCommandHandler(
+            dbContext,
+            _notifier,
+            _telegramNotifier,
+            currentUser
+        );
+
+        var result = await handler.Handle(
+            new CloseTicketCommand(ticket.Id),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(TicketStatus.Closed, ticket.Status);
+    }
+}

backend/tests/PnvPanel.Application.Tests/Admin/Support/RejectRoleRequestCommandHandlerTests.cs

@@ -0,0 +1,67 @@
+using NSubstitute;
+using PnvPanel.Application.Admin.Support;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Tests.TestSupport;
+using PnvPanel.Domain.Support;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Admin.Support;
+
+public class RejectRoleRequestCommandHandlerTests
+{
+    private readonly IRealtimeNotifier _notifier = Substitute.For<IRealtimeNotifier>();
+    private readonly ITelegramNotifier _telegramNotifier = Substitute.For<ITelegramNotifier>();
+
+    [Fact]
+    public async Task Handle_RejectsRoleRequest_ClosesTicket()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var userId = Guid.NewGuid();
+        var ticket = SupportTicket.CreateRoleRequestForExistingRole(userId, Guid.NewGuid());
+        dbContext.SupportTickets.Add(ticket);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var currentUser = FakeCurrentUser.Authenticated(Guid.NewGuid(), "admin");
+        var handler = new RejectRoleRequestCommandHandler(
+            dbContext,
+            _notifier,
+            _telegramNotifier,
+            currentUser
+        );
+
+        var result = await handler.Handle(
+            new RejectRoleRequestCommand(ticket.Id, "не подходит"),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(TicketStatus.Closed, ticket.Status);
+    }
+
+    [Fact]
+    public async Task Handle_WhenTicketIsOwnedByAdmin_StillRejects()
+    {
+        // Отклонить свою же заявку можно — Reject не меняет роль, риска нет.
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var adminId = Guid.NewGuid();
+        var ticket = SupportTicket.CreateRoleRequestForExistingRole(adminId, Guid.NewGuid());
+        dbContext.SupportTickets.Add(ticket);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var currentUser = FakeCurrentUser.Authenticated(adminId, "admin");
+        var handler = new RejectRoleRequestCommandHandler(
+            dbContext,
+            _notifier,
+            _telegramNotifier,
+            currentUser
+        );
+
+        var result = await handler.Handle(
+            new RejectRoleRequestCommand(ticket.Id, null),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(TicketStatus.Closed, ticket.Status);
+    }
+}

backend/tests/PnvPanel.Application.Tests/Admin/Support/ResolveTicketCommandHandlerTests.cs

@@ -0,0 +1,68 @@
+using NSubstitute;
+using PnvPanel.Application.Admin.Support;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Tests.TestSupport;
+using PnvPanel.Domain.Support;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Admin.Support;
+
+public class ResolveTicketCommandHandlerTests
+{
+    private readonly IRealtimeNotifier _notifier = Substitute.For<IRealtimeNotifier>();
+    private readonly ITelegramNotifier _telegramNotifier = Substitute.For<ITelegramNotifier>();
+
+    [Fact]
+    public async Task Handle_ResolvesOpenTicket()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var userId = Guid.NewGuid();
+        var ticket = SupportTicket.CreateBugReport(userId);
+        dbContext.SupportTickets.Add(ticket);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var currentUser = FakeCurrentUser.Authenticated(Guid.NewGuid(), "admin");
+        var handler = new ResolveTicketCommandHandler(
+            dbContext,
+            _notifier,
+            _telegramNotifier,
+            currentUser
+        );
+
+        var result = await handler.Handle(
+            new ResolveTicketCommand(ticket.Id),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(TicketStatus.Resolved, ticket.Status);
+    }
+
+    [Fact]
+    public async Task Handle_WhenTicketIsOwnedByAdmin_StillResolves()
+    {
+        // Решить свой же тикет можно — Resolve не меняет роль, риска нет (иначе тикет одинокого
+        // админа навсегда застревал бы в Open).
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var adminId = Guid.NewGuid();
+        var ticket = SupportTicket.CreateBugReport(adminId);
+        dbContext.SupportTickets.Add(ticket);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        var currentUser = FakeCurrentUser.Authenticated(adminId, "admin");
+        var handler = new ResolveTicketCommandHandler(
+            dbContext,
+            _notifier,
+            _telegramNotifier,
+            currentUser
+        );
+
+        var result = await handler.Handle(
+            new ResolveTicketCommand(ticket.Id),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        Assert.Equal(TicketStatus.Resolved, ticket.Status);
+    }
+}