fix(serve/tests): shut down AsyncioRouter after each test to prevent resource leaks

Per code review feedback: each AsyncioRouter instance spawns background
tasks (MetricsPusher, LongPollClient).  Without explicit teardown these
tasks leak into subsequent tests.  Add the same cleanup pattern used by
the existing setup_router fixture:

    await router.shutdown()
    router.long_poll_client.stop()

at the end of all three test cases in TestUpdateDeploymentConfigNullRouter.

Signed-off-by: chenshi5012 <chenshi5012@163.com>

Author: chenshi5012

python/ray/serve/tests/unit/test_router.py

@@ -1904,6 +1904,8 @@ async def test_update_deployment_config_with_null_router_no_exception(self):
         )
         # Should not raise AttributeError.
         router.update_deployment_config(DeploymentConfig())
+        await router.shutdown()
+        router.long_poll_client.stop()
 
     async def test_update_deployment_config_with_null_router_passes_zero_replicas(self):
         """When request_router is None, curr_num_replicas forwarded to
@@ -1937,6 +1939,8 @@ def _capture(deployment_config, curr_num_replicas):
             f"Expected 0 replicas when router is None, "
             f"got {captured['curr_num_replicas']}"
         )
+        await router.shutdown()
+        router.long_poll_client.stop()
 
     async def test_update_deployment_config_with_initialized_router_passes_correct_count(
         self,
@@ -1976,6 +1980,8 @@ def _capture(deployment_config, curr_num_replicas):
         assert captured["curr_num_replicas"] == 1, (
             f"Expected 1 replica, got {captured['curr_num_replicas']}"
         )
+        await router.shutdown()
+        router.long_poll_client.stop()
 
 
 @pytest.mark.asyncio