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.IntegrationTests/TestSupport/FakeXuiPanelGateway.cs

@@ -38,6 +38,7 @@ public sealed class FakeXuiPanelGateway : IXuiPanelGateway
         string clientEmail,
         string clientName,
         int limitIp,
+        DateTimeOffset? expiresAt,
         CancellationToken cancellationToken
     ) => Task.FromResult(Result.Success(Guid.NewGuid().ToString()));
 
@@ -54,8 +55,9 @@ public sealed class FakeXuiPanelGateway : IXuiPanelGateway
         string inboundRemoteId,
         string clientExternalId,
         VpnProtocol protocol,
-        string name,
-        bool enable,
+        string? name,
+        bool? enable,
+        DateTimeOffset? expiresAt,
         CancellationToken cancellationToken
     ) => Task.FromResult(Result.Success());