fix: Clear chat history when changing LLM models

- Modified on_model_change() to return empty list instead of None
- Changed output reference from chatbot.chatbot to chatbot.chatbot_value
- Ensures chat history is completely cleared when user switches models
- Prevents old conversation from being passed to newly selected model

Fixes the issue where chat history persisted across model changes.
Now provides proper conversation reset for clean model switching.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app.py

@@ -234,12 +234,13 @@ with gr.Blocks(
         current_model = new_model
         # Preload new model
         load_model(new_model)
-        return None
+        # Return empty list to clear chat history
+        return []
 
     model_dropdown.change(
         fn=on_model_change,
         inputs=[model_dropdown],
-        outputs=[chatbot.chatbot],
+        outputs=[chatbot.chatbot_value],
     )
 
     gr.Markdown(