Refactor client update handling to support nullable parameters for name and expiration

- Updated the `UpdateClientAsync` method in `IXuiPanelGateway` to accept nullable parameters for `name` and `expiresAt`, allowing for more flexible client management without unintended modifications.
- Adjusted the `BlockUserCommandHandler`, `UnblockUserCommandHandler`, and other related command handlers to utilize the new nullable parameters, ensuring that client names remain unchanged during block/unblock operations and that expiration dates are managed correctly.
- Enhanced the billing and configuration handling to reflect the new logic for managing client states based on expiration rather than enabling/disabling, improving reliability in client status management.
- Updated tests to cover the new behavior and ensure proper functionality across the application.

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

@@ -167,7 +167,8 @@ public class ConfirmPaymentRequestCommandHandlerTests
                 Arg.Any<string>(),
                 Arg.Any<VpnProtocol>(),
                 Arg.Any<string>(),
-                enable: true,
+                Arg.Any<bool?>(),
+                Arg.Any<DateTimeOffset?>(),
                 Arg.Any<CancellationToken>()
             )
             .Returns(Result.Success());
@@ -192,6 +193,8 @@ public class ConfirmPaymentRequestCommandHandlerTests
         Assert.NotNull(expiredConfig.ExpiresAt);
         Assert.NotNull(activeConfig.ExpiresAt);
         Assert.Equal(expiredConfig.ExpiresAt, activeConfig.ExpiresAt);
+        // enable: null — возврат из приостановки теперь идёт через expiresAt (новый newPaidUntil),
+        // а не через переключение enable (см. IXuiPanelGateway.UpdateClientAsync).
         await _gateway
             .Received(1)
             .UpdateClientAsync(
@@ -200,7 +203,8 @@ public class ConfirmPaymentRequestCommandHandlerTests
                 "ext-1",
                 VpnProtocol.Vless,
                 Arg.Any<string>(),
-                enable: true,
+                null,
+                Arg.Is<DateTimeOffset?>(d => d == expiredConfig.ExpiresAt),
                 Arg.Any<CancellationToken>()
             );
     }