fix #326 refresh naming cache on subscribe

[Sunrisea]

Summary

• Fix list_instances(subscribe=True) returning stale local naming cache when the service was loaded from disk but not subscribed to the server yet.
• Align Python SDK behavior with Java client semantics: subscribe=true confirms subscription and refreshes cache, while subscribe=false queries the server directly.
• Keep local cache fallback when subscribing from the server fails with a Nacos error.

Test plan

• ./.venv/bin/python -m unittest test.test_naming_service_cache -v
• ./.venv/bin/python -m unittest test.test_skill_and_prompt test.test_naming_service_cache -v
• ./.venv/bin/python -m compileall v2/nacos/naming test/test_naming_service_cache.py
• Local Nacos smoke test: stale disk cache with one real server instance refreshes correctly for subscribe=True and direct server query works for subscribe=False
• Local Nacos smoke test: batch-register two instances, batch-deregister one instance, subscribed cache updates to one instance via server push

Closes #326