Summary

Fixes #5450 - Race condition in localfilesystem context store that can cause data loss during shutdown.

May also help with #2723 (localFileSystem data cleared after power failure).

Changes

1. Added closing flag that is set at the start of close()
2. Check this flag in set() to prevent scheduling new timeouts during shutdown
3. Always flush pending writes in close() regardless of timeout state

Test Plan

• All existing context store tests pass
• Manual testing with rapid shutdown during writes