fix: Stop button actually cancels run_ui generator (not the .then lambda)

run_event was captured from .click().then(), so it pointed at the chained
0ms textbox-clear lambda. stop_btn.cancels=[run_event] was therefore
cancelling that no-op while the actual research generator kept running.
Capture .click() into run_event first, then chain .then() separately.

app.py

@@ -2600,13 +2600,19 @@ with gr.Blocks(
 """
     )
 
+    # IMPORTANT: keep `run_event` pointing at the .click() (the long-running
+    # generator), not at the chained .then(). stop_btn.cancels=[run_event]
+    # must target the generator for Stop to actually interrupt it; if we
+    # capture the .then() result instead, Stop cancels the instant clear
+    # lambda and the agent keeps running.
     run_event = run_btn.click(
         fn=run_ui,
         inputs=[question, max_turns, memory_strategy, temperature],
         outputs=[answer, trace],
-    ).then(
-        # Auto-clear the question textbox once the run finishes so the user
-        # can immediately ask a follow-up without manually pressing Clear.
+    )
+    run_event.then(
+        # Auto-clear the question textbox once the run finishes (or is
+        # cancelled) so the user can immediately ask a follow-up.
         fn=lambda: "",
         inputs=[],
         outputs=[question],