fix(service): skip empty wake-up runs

[Premsenareddy]

Summary

• pre-drain inbox content for wake-up driven ChatService runs
• skip agent execution when a wake-up finds no pending inbox entries
• add regression coverage for empty and non-empty wake-up runs

Tests

• .venv/bin/python -m pytest -p no:cacheprovider tests/service_wakeup_dispatcher_test.py tests/service_inbox_middleware_test.py tests/service_chat_wakeup_test.py -q

Addresses #1868