fix: add pragma no cover to Protocol stubs and unreachable mock branch

Protocol stub methods (fetch_token, create_authorization_url,
ensure_active_token) have Ellipsis bodies that coverage flags as
uncovered branches because Protocol classes are never instantiated.
Mark them with pragma: no cover.

The fallback path in _mock_iterdir (returning original_iterdir when the
path does not contain "fake") is only reachable on Windows where
jsonschema lazily loads schemas during the test. On Linux/macOS jsonschema
loads schemas eagerly so the branch is never taken. Mark it accordingly.

Author: harsh543

src/mcp/client/auth/authlib_adapter.py

@@ -64,11 +64,11 @@ class _AsyncOAuth2ClientProtocol(Protocol):
     scope: str | None
     code_challenge_method: str
 
-    async def fetch_token(self, url: str, **kwargs: Any) -> dict[str, Any]: ...
+    async def fetch_token(self, url: str, **kwargs: Any) -> dict[str, Any]: ...  # pragma: no cover
 
-    def create_authorization_url(self, url: str, **kwargs: Any) -> tuple[str, str]: ...
+    def create_authorization_url(self, url: str, **kwargs: Any) -> tuple[str, str]: ...  # pragma: no cover
 
-    async def ensure_active_token(self, token: dict[str, Any]) -> None: ...
+    async def ensure_active_token(self, token: dict[str, Any]) -> None: ...  # pragma: no cover
 
 
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