The cancellation handler previously promoted any partial send
(already_sent=True) to final_response_sent=True unconditionally.
This meant if intermediate text (e.g. 'Let me search…') was streamed
and the consumer was cancelled before delivering the actual answer,
the gateway's suppression check would still prevent the fallback send.
Now final_response_sent is only set in the cancellation path when:
- The best-effort send of accumulated content actually succeeded, OR
- It was already confirmed before cancellation
Companion fix for PR #11000's run.py changes — closes the
cancellation-path loophole that would otherwise let partial streams
suppress final delivery during queued follow-ups.
3b5572ded3