fix(browser): suppress KeyboardInterrupt in atexit cleanup thread stop

A second Ctrl+C while the atexit hook joins the browser janitor thread
surfaced as "Exception ignored in atexit callback: _stop_browser_cleanup_thread"
with a KeyboardInterrupt traceback. The janitor is a daemon thread and the
interpreter is already exiting, so nothing is lost by swallowing the
interrupt — the terminal tool's sibling _stop_cleanup_thread already does.

Salvaged from #10765 (function has since moved to browser_tool_lifecycle.py).

Fixes #10764
Co-authored-by: LehaoLin <lehaolin98@outlook.com>

tests/tools/test_browser_cleanup.py

@@ -187,3 +187,19 @@ class TestInactivityJanitorMultiplex:
         assert "t1" not in self.bt._active_sessions
         assert "t1" not in self.bt._session_last_activity
         assert "t1" not in self.bt._cleanup_failures
+
+
+class TestAtexitStopSwallowsInterrupt:
+    def test_second_ctrl_c_during_join_does_not_propagate(self, monkeypatch):
+        """A second Ctrl+C while the atexit hook waits on the janitor must not escape as a
+        traceback (#10764): the thread is a daemon, the interpreter is already exiting."""
+        from tools import browser_tool
+
+        class _InterruptedJoin:
+            def join(self, timeout=None):
+                raise KeyboardInterrupt
+
+        monkeypatch.setattr(browser_tool, "_cleanup_thread", _InterruptedJoin())
+        monkeypatch.setattr(browser_tool, "_cleanup_running", True)
+        bt_lifecycle._stop_browser_cleanup_thread()  # must not raise
+        assert browser_tool._cleanup_running is False

tools/browser_tool_lifecycle.py

@@ -424,7 +424,12 @@ def _stop_browser_cleanup_thread():
     """Stop the background cleanup thread."""
     _bt._cleanup_running = False
     if _bt._cleanup_thread is not None:
-        _bt._cleanup_thread.join(timeout=5)
+        # A second Ctrl+C during the timed join lands here as KeyboardInterrupt; the janitor is a
+        # daemon thread, so letting it propagate only prints "Exception ignored in atexit callback".
+        try:
+            _bt._cleanup_thread.join(timeout=5)
+        except (SystemExit, KeyboardInterrupt):
+            pass
 
 
 def _update_session_activity(task_id: str):