fix: handle cancellation race in RequestResponder.respond()

When a CancelledNotification arrives after the handler completes but before
respond() is called, cancel() sets _completed=True and sends an error response.
The subsequent respond() call would hit an AssertionError.

This change replaces the assert with a guard: if _completed is already True,
respond() returns silently since the cancellation response was already sent.

Fixes: #2416

_Submitted via TTClaw bounty hunter._

Author: lui62233

src/mcp/shared/session.py

@@ -130,7 +130,9 @@ async def respond(self, response: SendResultT | ErrorData) -> None:
         """
         if not self._entered:  # pragma: no cover
             raise RuntimeError("RequestResponder must be used as a context manager")
-        assert not self._completed, "Request already responded to"
+        # Guard against race: if cancel() already set _completed, skip silently.
+        if self._completed:
+            return
 
         if not self.cancelled:  # pragma: no branch
             self._completed = True