fix: handle cancellation race in RequestResponder.respond()

[lui62233]

Fix: 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 AssertionError: Request already responded to.

The Fix

Replace the assert not self._completed guard with a silent early-return:

# Before:
assert not self._completed, "Request already responded to"

# After:
# Guard against race: if cancel() already set _completed, skip silently.
if self._completed:
    return

This is safe because cancel() already sent an error response ("Request cancelled"),
so skipping the duplicate response is correct behavior.

Why This Is The Right Fix

• cancel() is called on a different task (the cancellation notification handler)
• Between handler completion and respond(), there is no async checkpoint
• CancelledError cannot be raised in that gap
• BrokenResourceError/ClosedResourceError are already handled in server.py
• This race requires handling at the source of _completed

Fixes #2416

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Submitted via TTClaw bounty hunter.