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 57.23%. Comparing base (bbb80be) to head (77787d2).

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         55.56%   57.23%   +1.66%     
================================================
  Files                40       41       +1     
  Lines              6117     6243     +126     
  Branches           1095     1095              
================================================
+ Hits               3399     3573     +174     
+ Misses             2202     2151      -51     
- Partials            516      519       +3     

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