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/Configs/Rotate/RotateVpnConfigCommandHandlerTests.cs

@@ -65,6 +65,7 @@ public class RotateVpnConfigCommandHandlerTests
                 Arg.Any<string>(),
                 Arg.Any<string>(),
                 Arg.Any<int>(),
+                Arg.Any<DateTimeOffset?>(),
                 Arg.Any<CancellationToken>()
             )
             .Returns(Result.Success("new-external-id"));
@@ -209,6 +210,7 @@ public class RotateVpnConfigCommandHandlerTests
                 Arg.Any<string>(),
                 Arg.Any<string>(),
                 Arg.Any<int>(),
+                Arg.Any<DateTimeOffset?>(),
                 Arg.Any<CancellationToken>()
             )
             .Returns(Result.Failure<string>(gatewayError));