tests(_internal[file_lock]): Add comprehensive tests for file locking module

why: Ensure file_lock module works correctly with thorough test coverage.

what:
- Add TestFileLock class with sync lock tests (acquire, release, reentrant)
- Add TestFileLockStaleDetection with parametrized stale lock scenarios
- Add TestAsyncFileLock with async context manager and timeout tests
- Add TestFileLockContext for dataclass state tests
- Add TestExceptions for picklability and message format
- Add TestAtomicInit and TestAsyncAtomicInit for two-file pattern
- Use NamedTuple+test_id parametrize pattern per project conventions
- All 40 tests pass including concurrent and threading scenarios

Author: tony