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)
 

tests/gateway/test_stream_consumer.py

@@ -599,3 +599,84 @@ class TestInterimCommentaryMessages:
         assert sent_texts == ["Hello ▉", "world"]
         assert consumer.already_sent is True
         assert consumer.final_response_sent is True
+
+
+class TestCancelledConsumerSetsFlags:
+    """Cancellation must set final_response_sent when already_sent is True.
+
+    The 5-second stream_task timeout in gateway/run.py can cancel the
+    consumer while it's still processing.  If final_response_sent stays
+    False, the gateway falls through to the normal send path and the
+    user sees a duplicate message.
+    """
+
+    @pytest.mark.asyncio
+    async def test_cancelled_with_already_sent_marks_final_response_sent(self):
+        """Cancelling after content was sent should set final_response_sent."""
+        adapter = MagicMock()
+        adapter.send = AsyncMock(
+            return_value=SimpleNamespace(success=True, message_id="msg_1")
+        )
+        adapter.edit_message = AsyncMock(
+            return_value=SimpleNamespace(success=True)
+        )
+        adapter.MAX_MESSAGE_LENGTH = 4096
+
+        consumer = GatewayStreamConsumer(
+            adapter,
+            "chat_123",
+            StreamConsumerConfig(edit_interval=0.01, buffer_threshold=5),
+        )
+
+        # Stream some text — the consumer sends it and sets already_sent
+        consumer.on_delta("Hello world")
+        task = asyncio.create_task(consumer.run())
+        await asyncio.sleep(0.08)
+
+        assert consumer.already_sent is True
+
+        # Cancel the task (simulates the 5-second timeout in gateway)
+        task.cancel()
+        try:
+            await task
+        except asyncio.CancelledError:
+            pass
+
+        # The fix: final_response_sent should be True even though _DONE
+        # was never processed, preventing a duplicate message.
+        assert consumer.final_response_sent is True
+
+    @pytest.mark.asyncio
+    async def test_cancelled_without_any_sends_does_not_mark_final(self):
+        """Cancelling before anything was sent should NOT set final_response_sent."""
+        adapter = MagicMock()
+        adapter.send = AsyncMock(
+            return_value=SimpleNamespace(success=False, message_id=None)
+        )
+        adapter.edit_message = AsyncMock(
+            return_value=SimpleNamespace(success=True)
+        )
+        adapter.MAX_MESSAGE_LENGTH = 4096
+
+        consumer = GatewayStreamConsumer(
+            adapter,
+            "chat_123",
+            StreamConsumerConfig(edit_interval=0.01, buffer_threshold=5),
+        )
+
+        # Send fails — already_sent stays False
+        consumer.on_delta("x")
+        task = asyncio.create_task(consumer.run())
+        await asyncio.sleep(0.08)
+
+        assert consumer.already_sent is False
+
+        task.cancel()
+        try:
+            await task
+        except asyncio.CancelledError:
+            pass
+
+        # Without a successful send, final_response_sent should stay False
+        # so the normal gateway send path can deliver the response.
+        assert consumer.final_response_sent is False