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/src/PnvPanel.Application/Admin/Users/BlockUserCommandHandler.cs

@@ -42,13 +42,17 @@ public sealed class BlockUserCommandHandler(
 
             if (inbound is not null && node is not null)
             {
+                // name: null — не трогаем текущее имя клиента в панели, это не переименование.
+                // expiresAt: null — блокировка админом отдельная ось от биллинга (см.
+                // IXuiPanelGateway.UpdateClientAsync), срок оплаты не трогаем.
                 var updateResult = await gateway.UpdateClientAsync(
                     node,
                     inbound.RemoteInboundId,
                     config.ClientExternalId,
                     config.Protocol,
-                    config.Label ?? config.ClientEmail,
+                    name: null,
                     enable: false,
+                    expiresAt: null,
                     cancellationToken
                 );