fix: drop unsupported show_copy_button kwarg on Textbox (Gradio 6)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app.py

@@ -746,7 +746,6 @@ with gr.Blocks(fill_height=True) as demo:
                     vlm_output = gr.Textbox(
                         label="🤖 Answer",
                         lines=20,
-                        show_copy_button=True,
                     )
 
     gr.Markdown(