Improve coverage for file operations

[soulteary]

Summary

• allow overriding filesystem helpers in fn package to make error paths testable
• add targeted tests for file handling and directory preparation edge cases
• ensure GetPathContent, Save, and ensureDirectory reach 100% coverage

Testing

• go test ./...
• go test ./internal/fn -cover

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https://chatgpt.com/codex/tasks/task_e_690cc949ad408327bd10df8a3b570296

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Note

Make filesystem helpers overridable and add targeted tests for failure paths in GetPathContent, Save, and ensureDirectory.

• fn package:
  • Introduce overridable filesystem helpers: readFile, writeFile, stat, mkdirAll.
  • Replace direct os calls with helpers in GetPathContent, Save, and ensureDirectory.
• Tests:
  • Add internal/fn/fn_errors_test.go covering:
    • Read failure in GetPathContent.
    • Save failures: stat error, writeFile error.
    • ensureDirectory edge cases: existing file (not dir), parent not a dir, mkdirAll error, unexpected stat error.

^{Written by Cursor Bugbot for commit 8b55829. This will update automatically on new commits. Configure here.}

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (b04ac66) to head (8b55829).
⚠️ Report is 5 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
+ Coverage   96.63%   98.99%   +2.35%     
==========================================
  Files          11       11              
  Lines         595      595              
==========================================
+ Hits          575      589      +14     
+ Misses         10        3       -7     
+ Partials       10        3       -7     

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