fix(gateway): scope audio_file_paths outside media_urls guard

The audio-file-paths handling block at line 7334 references the variable
unconditionally, but #24879 initialized it inside the 'if event.media_urls'
block — so events without media_urls hit UnboundLocalError.

Found via test_run_agent_queued_message_does_not_treat_commentary_as_final
after PR #28478 landed.

gateway/run.py

@@ -7254,10 +7254,13 @@ class GatewayRunner:
         if getattr(event, "channel_context", None):
             message_text = f"{event.channel_context}\n\n[New message]\n{message_text}"
 
+        # Declare at outer scope so the audio-file-paths handling block below
+        # remains safe when ``event.media_urls`` is empty (no inner block runs).
+        audio_file_paths: list[str] = []
+
         if event.media_urls:
             image_paths = []
             audio_paths = []
-            audio_file_paths: list[str] = []  # audio file attachments — not for STT
             for i, path in enumerate(event.media_urls):
                 mtype = event.media_types[i] if i < len(event.media_types) else ""
                 if mtype.startswith("image/") or event.message_type == MessageType.PHOTO: