Two related problems:
- During long AIHelper calls (non-streaming LLM), no data flows to the
  WebSocket and browsers drop the connection after ~30-60s of inactivity.
  Fixed with a 20s heartbeat: _stream_to_client now uses asyncio.wait_for
  and sends {"type":"heartbeat"} on timeout to keep the connection alive.

- After reconnect, if the agent finished while the client was offline,
  _runs no longer holds the session and no stream_start is sent. Client
  would reconnect silently with no response shown. Fixed by sending
  {"type":"session_sync"} on every new WS connection (after reattach
  completes or immediately when no run is active) so the client knows
  to reload session history.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>