fix(streaming): prevent duplicate Telegram replies when stream task is cancelled (#9319)

When the 5-second stream_task timeout in gateway/run.py expires (due to
slow Telegram API calls from rate limiting after several messages), the
stream consumer is cancelled via asyncio.CancelledError. The
CancelledError handler did a best-effort final edit but never set
final_response_sent, so the gateway fell through to the normal send path
and delivered the full response again as a reply — causing a duplicate.

The fix: in the CancelledError handler, set final_response_sent = True
when already_sent is True (i.e., the stream consumer had already
delivered content to the user). This tells the gateway's already_sent
check that the response was delivered, preventing the duplicate send.

Adds two tests verifying the cancellation behavior:
- Cancelled with already_sent=True → final_response_sent=True (no dup)
- Cancelled with already_sent=False → final_response_sent=False (normal
  send path proceeds)

Reported by community user hume on Discord.

gateway/stream_consumer.py

@@ -280,6 +280,14 @@ class GatewayStreamConsumer:
                     await self._send_or_edit(self._accumulated)
                 except Exception:
                     pass
+            # If we delivered any content before being cancelled, mark the
+            # final response as sent so the gateway's already_sent check
+            # doesn't trigger a duplicate message.  The 5-second
+            # stream_task timeout (gateway/run.py) can cancel us while
+            # waiting on a slow Telegram API call — without this flag the
+            # gateway falls through to the normal send path.
+            if self._already_sent:
+                self._final_response_sent = True
         except Exception as e:
             logger.error("Stream consumer error: %s", e)