fix: ClientController CreateEndpoint type mismatch

FIX:
- Remove invalid null-coalescing operator between Client and CreateClientDto types
- Use null-forgiving operator (!) since created client is immediately retrieved
- Ensure type safety while preserving nullable reference semantics

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/TaxBaik.Web/Endpoints/Client/CreateEndpoint.cs

@@ -24,8 +24,8 @@ public class CreateEndpoint : Endpoint<CreateClientDto, object>
         try
         {
             var clientId = await _clientService.CreateAsync(request);
-            var client = await _clientService.GetByIdAsync(clientId) ?? request;
-            await SendAsync(client, 201, cancellation: ct);
+            var client = await _clientService.GetByIdAsync(clientId);
+            await SendAsync(client!, 201, cancellation: ct);
         }
         catch (ValidationException ex)
         {