fix(stream-consumer): only confirm final delivery on successful best-effort send

The cancellation handler previously promoted any partial send
(already_sent=True) to final_response_sent=True unconditionally.
This meant if intermediate text (e.g. 'Let me search…') was streamed
and the consumer was cancelled before delivering the actual answer,
the gateway's suppression check would still prevent the fallback send.

Now final_response_sent is only set in the cancellation path when:
- The best-effort send of accumulated content actually succeeded, OR
- It was already confirmed before cancellation

Companion fix for PR #11000's run.py changes — closes the
cancellation-path loophole that would otherwise let partial streams
suppress final delivery during queued follow-ups.

gateway/stream_consumer.py

@@ -403,18 +403,20 @@ class GatewayStreamConsumer:
 
         except asyncio.CancelledError:
             # Best-effort final edit on cancellation
+            _best_effort_ok = False
             if self._accumulated and self._message_id:
                 try:
-                    await self._send_or_edit(self._accumulated)
+                    _best_effort_ok = bool(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:
+            # Only confirm final delivery if the best-effort send above
+            # actually succeeded OR if the final response was already
+            # confirmed before we were cancelled.  Previously this
+            # promoted any partial send (already_sent=True) to
+            # final_response_sent — which suppressed the gateway's
+            # fallback send even when only intermediate text (e.g.
+            # "Let me search…") had been delivered, not the real answer.
+            if _best_effort_ok and not self._final_response_sent:
                 self._final_response_sent = True
         except Exception as e:
             logger.error("Stream consumer error: %s", e)