Fix get_state_names omitting untouched initstate keys in MockStateManager

[magic-peach]

Description

MockStateManager.get_state_names() only looked at _default_state_change_tracker, which stays empty for any state key that was seeded via initstate and never explicitly read or written during the current test. contains_state() right next to it already falls back to _mock_state (the dict MockActor's own docstring shows users asserting against directly), so the two methods disagreed: contains_state('balance') returned True for a seeded key while get_state_names() silently omitted it.

Since every mutating method (try_add_state, set_state_ttl, try_remove_state, get_or_add_state, add_or_update_state) already keeps _mock_state in sync, get_state_names() now just returns list(self._mock_state.keys()).

Issue reference

Please reference the issue this PR will close: N/A, self-discovered while testing actor state mocking, no existing issue filed.

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation