chore: introduce better test coverage

[kdihalas]

This pull request adds comprehensive tests for the wtcache package, focusing on file system operation behaviors, error propagation, and cache synchronization. It introduces new test files and extends the failingFs helper to simulate failures for a wider range of file system operations. The most important changes are:

Expanded error simulation for testing:

• The failingFs struct is updated to support forced failures for operations like Remove, RemoveAll, Rename, Mkdir, Chmod, Chown, and Chtimes, enabling more thorough error-propagation tests.

New and extended test coverage:

• cache_more_test.go: Adds a large suite of tests covering cache behavior for directory creation, file operations, error propagation, cache invalidation, and synchronization between memory and remote file systems. This includes tests for Mkdir, MkdirAll, OpenFile, RemoveAll, Stat, Chmod, Chown, Chtimes, Rename, and more.
• file_test.go: Introduces tests for the cacheFile type, specifically ensuring that Close behaves correctly for both writable and non-writable files, and that underlying file close errors are properly handled without unwanted side effects.

Test infrastructure improvements:

• Implements helper types like errCloseFile to simulate file close errors for robust negative testing scenarios.

These changes significantly improve the reliability and coverage of the test suite for the write-through cache file system implementation.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.08%. Comparing base (6e5ccd9) to head (6c86086).
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Additional details and impacted files

@@             Coverage Diff             @@
##             main       #4       +/-   ##
===========================================
+ Coverage   46.93%   84.08%   +37.14%     
===========================================
  Files           4        4               
  Lines         245      245               
===========================================
+ Hits          115      206       +91     
+ Misses        116       32       -84     
+ Partials       14        7        -7     

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