Update package versions and improve null safety in tests

- Updated various package versions in `Directory.Packages.props` to the latest compatible releases, enhancing overall stability and security.
- Modified test assertions in `GrantBillingGiftCommandHandlerTests`, `RejectPaymentRequestCommandHandlerTests`, `BlockUserCommandHandlerTests`, and `DeleteUserCommandHandlerTests` to use null-safe checks, ensuring robustness against potential null reference exceptions.
- Updated PostgreSqlContainer initialization in `PnvPanelWebApplicationFactory` for improved clarity and maintainability.

backend/tests/PnvPanel.Application.Tests/Admin/Billing/GrantBillingGiftCommandHandlerTests.cs

@@ -74,7 +74,7 @@ public class GrantBillingGiftCommandHandlerTests
             .Received(1)
             .NotifyUserAsync(
                 userId,
-                Arg.Is<string>(m => m.Contains("30")),
+                Arg.Is<string>(m => m!.Contains("30")),
                 Arg.Any<string?>(),
                 Arg.Any<CancellationToken>()
             );

backend/tests/PnvPanel.Application.Tests/Admin/Billing/RejectPaymentRequestCommandHandlerTests.cs

@@ -71,7 +71,7 @@ public class RejectPaymentRequestCommandHandlerTests
             .Received(1)
             .NotifyUserAsync(
                 userId,
-                Arg.Is<string>(m => m.Contains("Платёж не найден")),
+                Arg.Is<string>(m => m!.Contains("Платёж не найден")),
                 Arg.Any<string?>(),
                 Arg.Any<CancellationToken>()
             );

backend/tests/PnvPanel.Application.Tests/Admin/Users/BlockUserCommandHandlerTests.cs

@@ -118,7 +118,7 @@ public class BlockUserCommandHandlerTests
         await _gateway
             .Received(1)
             .UpdateClientAsync(
-                Arg.Is<Node>(n => n.Id == node.Id),
+                Arg.Is<Node>(n => n!.Id == node.Id),
                 inbound.RemoteInboundId,
                 config.ClientExternalId,
                 config.Protocol,

backend/tests/PnvPanel.Application.Tests/Admin/Users/DeleteUserCommandHandlerTests.cs

@@ -99,7 +99,7 @@ public class DeleteUserCommandHandlerTests
         await _gateway
             .Received(1)
             .RemoveClientAsync(
-                Arg.Is<Node>(n => n.Id == node.Id),
+                Arg.Is<Node>(n => n!.Id == node.Id),
                 inbound.RemoteInboundId,
                 "external-id",
                 config.Protocol,