fix: remove stale pragma no cover from lines now covered by tests

strict-no-cover correctly identified that cancel() guards and
notification validation are now exercised by the new cancellation tests.
Remove the outdated pragmas per AGENTS.md guidelines.

Author: dragogargo

src/mcp/shared/session.py

@@ -142,9 +142,9 @@ async def respond(self, response: SendResultT | ErrorData) -> None:
 
     async def cancel(self) -> None:
         """Cancel this request and mark it as completed."""
-        if not self._entered:  # pragma: no cover
+        if not self._entered:
             raise RuntimeError("RequestResponder must be used as a context manager")
-        if not self._cancel_scope:  # pragma: no cover
+        if not self._cancel_scope:
             raise RuntimeError("No active cancel scope")
 
         self._cancel_scope.cancel()
@@ -423,7 +423,7 @@ async def _handle_session_message(message: SessionMessage) -> None:
                                 await self._handle_incoming(notification)
                         except Exception:
                             # For other validation errors, log and continue
-                            logging.warning(  # pragma: no cover
+                            logging.warning(
                                 f"Failed to validate notification:. Message was: {message.message}",
                                 exc_info=True,
                             )