fix: clarify cancelled chat turn status (Jordan-SkyLF)
Conflict resolution on api/streaming.py:4549-4567 (the cancel-handler
ownership guard). Both this PR and the already-shipped PR #2136 add a
guard at the same site against stale stream writebacks, from different
angles:
- PR #2136 (HEAD): _stream_writeback_is_current(_cs, stream_id) — strictly
dominates by checking the active_stream_id token equality.
- PR #2151: 'worker won the race' check via (active_stream_id != stream_id
and not pending_user_message), with _emit_cancel_event = False to suppress
the terminal cancel event.
Resolution merges both: keep #2136's strictly-stronger condition for skip
detection, and adopt #2151's _emit_cancel_event = False semantic so the
cancel event isn't emitted in addition to skipping the writeback (when
client may have already received the successful done payload).
55/55 tests pass across cancelled-turn-status + stale-stream-writeback +
the four cancel/data-loss sibling test files.
3f851051cf