feat(Type Cache Deprecation): Fast stat bucket cache implicit dir

[Tulsishah]

Description

In GCSFuse, an "implicit directory" is a folder that exists as a prefix to other objects but does not have a corresponding 0-byte object in GCS. Historically, these were tracked in a separate typeCache. This PR migrates that responsibility to the statCache by inferring directory existence from listing results.

Link to the issue in case of a bug fix.

b/473410044

Testing details

1. Manual - NA
2. Unit tests - Added
3. Integration tests - Automated

Any backward incompatible change? If so, please explain.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.12%. Comparing base (54895d1) to head (8e9ccb4).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/storage/caching/fast_stat_bucket.go	88.88%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4237      +/-   ##
==========================================
+ Coverage   83.08%   83.12%   +0.04%     
==========================================
  Files         154      154              
  Lines       18913    18973      +60     
==========================================
+ Hits        15714    15772      +58     
  Misses       2625     2625              
- Partials      574      576       +2     

Flag	Coverage Δ
unittests	83.12% <88.88%> (+0.04%)	⬆️

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[Tulsishah]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces new logic to handle implicit directories when caching GCS listings, controlled by a new IsTypeCacheDeprecated flag. The changes include a new insertListing function in fastStatBucket and a corresponding test suite. My review has identified a bug in the implicit directory detection logic, where directories containing only subdirectories are not correctly cached. I've also found an issue in the corresponding test case that fails to check for this, and a potential for test flakiness due to using map iteration for ordered mock expectations. I've provided suggestions to fix these issues.