Fix race condition: await remove_all_listeners() in on_shutdown

[NodeJSmith]

Summary

• Bus.remove_all_listeners() returns an asyncio.Task but three on_shutdown methods (ServiceWatcher, StateProxy, AppHandler) called it without await, creating a race condition where listeners could survive shutdown if the task bucket was cancelled first
• Added await to all three call sites and fixed the test fixture mock to return an awaitable task

Closes #309

[Copilot]

Pull request overview

This PR fixes a race condition in shutdown handlers where Bus.remove_all_listeners() was called without await, potentially allowing listeners to survive shutdown if the task bucket was cancelled before cleanup completed.

Changes:

• Added await to three remove_all_listeners() calls in on_shutdown() methods
• Fixed test fixture to return an awaitable Task instead of a plain Mock

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/hassette/core/state_proxy.py	Added await to remove_all_listeners() call in on_shutdown()
src/hassette/core/service_watcher.py	Added await to remove_all_listeners() call in on_shutdown()
src/hassette/core/app_handler.py	Added await to remove_all_listeners() call in on_shutdown()
tests/integration/test_state_proxy.py	Updated mock to return awaitable Task via asyncio.ensure_future()
CHANGELOG.md	Added entry documenting the fix

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.99%. Comparing base (9d0147c) to head (4cfda51).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #311   +/-   ##
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  Coverage   79.99%   79.99%           
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  Files         134      134           
  Lines        9531     9531           
  Branches      943      943           
=======================================
  Hits         7624     7624           
- Misses       1538     1539    +1     
+ Partials      369      368    -1     

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