Problem

When a CancelledNotification arrives after a request handler returns its result but before respond() is called, a race window exists in _handle_request:

1. cancel() sets _completed = True and sends an error response to the client
2. respond() is then called — and crashes with AssertionError: Request already responded to

This manifests as an unhandled exception in the server's task group, reported in #2416.

Root Cause

respond() used an assert to guard against double-respond:

assert not self._completed, "Request already responded to"

This is correct for programming errors (calling respond() twice) but wrong for the concurrent-cancellation case, where _completed = True was legitimately set by a racing cancel() call. The assertion doesn't distinguish between the two scenarios.

Fix

Replace the assert with an early return:

if self._completed:
    return

When _completed is already True due to a prior cancellation, respond() silently discards the late result — the request is already handled, so there is nothing useful to send.

Test

Added tests/server/test_respond_after_cancellation.py which simulates the race by directly setting _completed = True before calling respond(), verifying that no exception is raised.