test: remove covered oauth no-cover marker

Author: Genmin

src/mcp/client/auth/oauth2.py

@@ -624,7 +624,7 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                     self._copy_user_agent_header(token_request, request)
                     token_response = yield token_request
                     await self._handle_token_response(token_response)
-                except Exception:  # pragma: no cover
+                except Exception:
                     logger.exception("OAuth flow error")
                     raise