fix(backend/chat): Filter duplicate StreamStart from non-SDK path

Routes.py already publishes a StreamStart before calling the service.
The SDK path filters the duplicate internally, but the non-SDK path
did not, causing two StreamStart events to reach the frontend.

autogpt_platform/backend/backend/api/features/chat/routes.py

@@ -417,6 +417,9 @@ async def stream_chat_post(
                 session=session,  # Pass session with message already added
                 context=request.context,
             ):
+                # Skip duplicate StreamStart — we already published one above
+                if isinstance(chunk, StreamStart):
+                    continue
                 chunk_count += 1
                 if first_chunk_time is None:
                     first_chunk_time = time_module.perf_counter()