fix(gateway): use source.thread_id instead of undefined event in queued response

In _run_agent(), the pending message handler references 'event' which
is not defined in that scope — it only exists in the caller. This
causes a NameError when sending the first response before processing a
queued follow-up message.

Replace getattr(event, 'metadata', None) with the established pattern
using source.thread_id, consistent with lines 2625, 2810, 3678, 4410, 4566
in the same file.

gateway/run.py

@@ -8131,7 +8131,7 @@ class GatewayRunner:
                     if first_response and not _already_streamed:
                         try:
                             await adapter.send(source.chat_id, first_response,
-                                               metadata=getattr(event, "metadata", None))
+                                               metadata={"thread_id": source.thread_id} if source.thread_id else None)
                         except Exception as e:
                             logger.warning("Failed to send first response before queued message: %s", e)
                 # else: interrupted — discard the interrupted response ("Operation