Fix type error in FastMCP auth test

Author: LucaButBoring

tests/server/fastmcp/auth/test_auth_integration.py

@@ -39,6 +39,7 @@ async def get_client(self, client_id: str) -> OAuthClientInformationFull | None:
         return self.clients.get(client_id)
 
     async def register_client(self, client_info: OAuthClientInformationFull):
+        assert client_info.client_id is not None
         self.clients[client_info.client_id] = client_info
 
     async def authorize(self, client: OAuthClientInformationFull, params: AuthorizationParams) -> str: