feat(hns): remove empty-dir check before folder rename

[PranjalC100]

Description

This PR optimizes the renameHierarchicalDir operation for Hierarchical Namespace (HNS) buckets by removing the preliminary checkDirNotEmpty validation step.

Previously, GCSFuse performed a recursive listing of the destination directory via ReadDescendants (translating to a ListObjects call with MaxResults=2) to verify it was empty before attempting a rename.

By eliminating this redundant client-side check, we shift the validation burden to the native GCS backend, which inherently rejects the action if the destination is not empty. This saves a Class A List API operation, reducing latency and operational costs for customers, while maintaining strict POSIX compliance.

This implementation covers the "Code Changes" task outlined in the HNS Folder Rename Optimization design document.

Link to the issue in case of a bug fix.

https://b.corp.google.com/issues/495920968

Testing details

1. Manual - Ran custom Python os.rename tests against mounted HNS buckets with TRACE logs enabled. Verified that renaming to a non-empty directory natively fails and returns ENOTEMPTY without triggering any preliminary ListObjects calls.
2. Unit tests - Updated rename and directory validation tests to align with the removed check.
3. Integration tests - Ran existing HNS rename integration tests to ensure expected POSIX behavior is maintained.

Any backward incompatible change? If so, please explain.

No backward incompatible changes. The error propagation to the OS (e.g., ENOTEMPTY) remains functionally identical.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.65%. Comparing base (9651ba5) to head (f7305ea).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/fs/fs.go	50.00%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4614       +/-   ##
===========================================
+ Coverage        0   83.65%   +83.65%     
===========================================
  Files           0      164      +164     
  Lines           0    20216    +20216     
===========================================
+ Hits            0    16911    +16911     
- Misses          0     2668     +2668     
- Partials        0      637      +637     

Flag	Coverage Δ
unittests	83.65% <50.00%> (?)

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