test(oauth): mark invariant branches for coverage

Author: ctaylor86

src/mcp/client/auth/oauth2.py

@@ -666,7 +666,7 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                 # Retry with new tokens
                 self._add_auth_header(request)
                 retry_after_authorization = True
-            elif response.status_code == 403:
+            elif response.status_code == 403:  # pragma: no branch
                 # Step 1: Extract error field from WWW-Authenticate header
                 error = extract_field_from_www_auth(response, "error")
 
@@ -690,5 +690,5 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                 retry_after_authorization = True
 
         # The retried resource request can itself be a session-long GET.
-        if retry_after_authorization:
+        if retry_after_authorization:  # pragma: no branch
             yield request

tests/client/test_auth.py

@@ -2413,7 +2413,7 @@ async def complete_request() -> None:
     async with anyio.create_task_group() as task_group:
         async with oauth_provider.context.flow_lock:
             task_group.start_soon(complete_request)
-            with anyio.fail_after(5):
+            with anyio.fail_after(5):  # pragma: no branch
                 await completed.wait()
 
 
@@ -2441,7 +2441,7 @@ async def drive_403() -> None:
                 while oauth_provider.context.flow_lock.statistics().tasks_waiting == 0:
                     await anyio.sleep(0)
 
-            async with oauth_provider.context.lock:
+            async with oauth_provider.context.lock:  # pragma: no branch
                 oauth_provider.context.current_tokens = OAuthToken(
                     access_token="new_access_token",
                     token_type="Bearer",