fix: correct message ordering after task cancellation — v0.50.163 (#883)

fix: correct message ordering after task cancellation — v0.50.163 (#883)

Fixes the message-ordering glitch from #882: clicking Cancel while the
agent is responding could cause a subsequent response to render above
the "*Task cancelled.*" marker.

Root cause: the cancel handler pushed the marker only to local S.messages
without persisting to the server. When the done event fired shortly after
and replaced S.messages from server state, the marker disappeared from
client state while the next response anchored to the server-authoritative
position.

Fix has three parts:
- Server (cancel_stream): append *Task cancelled.* to session.messages
  with _error:True + timestamp, then save. _error ensures
  _sanitize_messages_for_api() strips it from conversation_history on
  the next agent turn, so the LLM never sees it as a prior assistant
  turn. Precedent: same flag used for the apperror marker at line 1343.
- Client (SSE cancel handler): fetch /api/session instead of pushing
  locally (same pattern as the done handler). Falls back to local push
  if the fetch fails.
- Tests: fix test window width for cancel handler (1200→dynamic); add
  two regression tests pinning _error flag and _sanitize invariant.

1941 tests passing.

Co-authored-by: piliang <piliang1@jd.com>

api/streaming.py

@@ -1666,6 +1666,18 @@ def cancel_stream(stream_id: str) -> bool:
             _cs.pending_user_message = None
             _cs.pending_attachments = []
             _cs.pending_started_at = None
+            # Add cancel message to session messages so client sees consistent state.
+            # _error=True flags this as a synthetic UI marker so
+            # _sanitize_messages_for_api() (line 591-593) strips it from the
+            # conversation_history passed to the agent on the NEXT user message —
+            # otherwise the model would see "Task cancelled." in its history as a
+            # prior assistant turn and could respond accordingly.
+            _cs.messages.append({
+                'role': 'assistant',
+                'content': '*Task cancelled.*',
+                '_error': True,
+                'timestamp': int(time.time()),
+            })
             _cs.save()
         except Exception:
             logger.debug("Failed to clear session state on cancel for %s", _cancel_session_id)