Fix Gemini streaming response not persisted when leaving chat

Add yield final_content at end of successful streaming to ensure
the async iterator completes properly and the response is persisted
even when the user navigates away during streaming.

Co-authored-by: owndev <69784886+owndev@users.noreply.github.com>

Author: Copilot

pipelines/google/google_gemini.py

@@ -2052,6 +2052,10 @@ async def emit_chat_event(event_type: str, data: Dict[str, Any]) -> None:
                 {"role": "assistant", "content": final_content, "done": True},
             )
 
+            # Yield final content to ensure the async iterator completes properly.
+            # This ensures the response is persisted even if the user navigates away.
+            yield final_content
+
         except Exception as e:
             self.log.exception(f"Error during streaming: {e}")
             # Check if it's a chunk size error and provide specific guidance