fix: keep question textbox intact when Stop cancels the run

.then() fires on any termination (success, error, cancel) — so Stop was
clearing the textbox along with everything else, forcing the user to
retype their question. Switching to .success() limits the auto-clear to
clean completions only.

app.py

@@ -2610,9 +2610,10 @@ with gr.Blocks(
         inputs=[question, max_turns, memory_strategy, temperature],
         outputs=[answer, trace],
     )
-    run_event.then(
-        # Auto-clear the question textbox once the run finishes (or is
-        # cancelled) so the user can immediately ask a follow-up.
+    # .success() (not .then()) so the textbox is cleared ONLY on a clean run.
+    # If Stop cancels the generator we leave the question intact, so the user
+    # can tweak it and re-run without retyping.
+    run_event.success(
         fn=lambda: "",
         inputs=[],
         outputs=[question],