ci: enable async fd leak detection in native test job

[paveg]

Summary

Enable moonbitlang/async's file-descriptor leak detection in the native test job.
With MOONBIT_ASYNC_CHECK_FD_LEAK=1 (available since async 0.19.1),
any program that exits with unclosed files or network connections managed by the async runtime aborts with an error,
turning fd leaks into hard CI failures instead of silent resource bugs.

Verification

Ran locally against main with the flag enabled:
MOONBIT_ASYNC_CHECK_FD_LEAK=1 moon test --target native → 256/256 passed, no leaks detected,
so this check starts from a green baseline.

🤖 Generated with Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: f725039

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.59%. Comparing base (8fbb433) to head (f725039).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   63.59%   63.59%           
=======================================
  Files          16       16           
  Lines         739      739           
=======================================
  Hits          470      470           
  Misses        269      269           

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