tests(_internal): Add async tests for file_lock module

[tony]

Summary

• Add async tests for AsyncFileLock and async_atomic_init
• Add pytest-asyncio dependency and configuration

Depends on #501.

Test plan

• 8 async tests for AsyncFileLock (context manager, acquire/release, reentrant, timeout, etc.)
• 4 async tests for async_atomic_init (first init, already done, sync fn, concurrent)

[codecov]

Codecov Report

❌ Patch coverage is 99.20635% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 61.43%. Comparing base (1bd57a4) to head (9456c94).

Files with missing lines	Patch %	Lines
tests/_internal/test_file_lock_async.py	99.20%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           pr/file-lock     #505      +/-   ##
================================================
+ Coverage         59.98%   61.43%   +1.45%     
================================================
  Files                42       43       +1     
  Lines              6930     7056     +126     
  Branches           1130     1130              
================================================
+ Hits               4157     4335     +178     
+ Misses             2235     2180      -55     
- Partials            538      541       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.