Fix session cancel and new task routing

tests/test_gui_server.py

@@ -349,6 +349,29 @@ class GuiServerTests(unittest.TestCase):
             self.assertIsNotNone(row)
             self.assertEqual(row['status'], 'cancelled')
 
+    def test_cancel_run_abandons_held_session_lock(self) -> None:
+        with tempfile.TemporaryDirectory() as d:
+            client, state = _build_client(Path(d))
+            record = state.run_manager.start('alice', 'thread-1')
+            old_lock = state.run_lock_for('alice', 'thread-1')
+            self.assertTrue(old_lock.acquire(blocking=False))
+            old_agent = state.agent_for('alice', 'thread-1')
+            try:
+                response = client.post(
+                    '/api/runs/cancel',
+                    json={'account_id': 'alice', 'session_id': 'thread-1'},
+                )
+                self.assertEqual(response.status_code, 200)
+                self.assertTrue(response.json()['cancelled'])
+                self.assertTrue(record.cancel_event.is_set())
+                new_lock = state.run_lock_for('alice', 'thread-1')
+                self.assertIsNot(new_lock, old_lock)
+                self.assertTrue(new_lock.acquire(blocking=False))
+                new_lock.release()
+                self.assertIsNot(state.agent_for('alice', 'thread-1'), old_agent)
+            finally:
+                old_lock.release()
+
     def test_latest_run_prefers_running_over_queued_for_session(self) -> None:
         with tempfile.TemporaryDirectory() as d:
             client, state = _build_client(Path(d))