fix(copilot): exclude heartbeat events from _events_yielded retry guard

StreamHeartbeat events are non-substantive keep-alive signals that
clients ignore. Counting them in _events_yielded incorrectly prevented
the retry-with-compaction logic from running when a tool call took
longer than 10 seconds before a prompt-too-long error.

autogpt_platform/backend/backend/copilot/sdk/service.py

@@ -1519,7 +1519,8 @@ async def stream_chat_completion_sdk(
 
             try:
                 async for event in _run_stream_attempt(state):
-                    _events_yielded += 1
+                    if not isinstance(event, StreamHeartbeat):
+                        _events_yielded += 1
                     yield event
                 break  # Stream completed — exit retry loop
             except asyncio.CancelledError: