refactor: use modern Go helpers

[rustatian]

Reason for This PR

Use standard Go helpers for worker shutdown and timeout checks.

Description of Changes

• Use sync.OnceFunc for watcher shutdown and the shared test cleanup.
• Use t.Context() and maps.Keys in the worker spawn-timeout test.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

PR Checklist

• All commits in this PR are signed (git commit -s -S).
• The reason for this PR is clearly provided.
• The description of changes is clear and encompassing.
• Any required documentation changes are included in this PR.
• The change preserves the public API and behavior.
• All added or changed functionality is tested.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.24%. Comparing base (43d99a7) to head (098484a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   77.13%   77.24%   +0.10%     
==========================================
  Files          20       20              
  Lines        1159     1160       +1     
==========================================
+ Hits          894      896       +2     
+ Misses        265      264       -1     

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