fix: skip chmod on existing directories in SaveUploadedFile

[Herrtian]

Summary

Fixes #4622

SaveUploadedFile called os.Chmod on the target directory unconditionally, even if it already existed. This broke file uploads to system directories like /tmp:

chmod /tmp: operation not permitted

Change

Added a check before calling os.Chmod: only newly created directories get their permissions set. Pre-existing directories are left untouched.

// Before: always chmod (breaks on /tmp, /var, etc.)
os.MkdirAll(dir, mode)
os.Chmod(dir, mode)  // fails if dir is /tmp

// After: only chmod new directories
dirExisted := dirExists(dir)
os.MkdirAll(dir, mode)
if !dirExisted {
    os.Chmod(dir, mode)
}

Updated TestSaveUploadedFileWithPermission to verify permissions on a newly created subdirectory rather than the current working directory. Added a Windows skip for the permission assertion since Windows doesn't support Unix file permissions.

Test plan

• go build ./... passes
• TestSaveUploadedFileWithPermission passes
• TestSaveUploadedFileWithPermissionFailed passes
• TestFileDescriptor failure is pre-existing (fails on master too on Windows)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.44%. Comparing base (3dc1cd6) to head (2ac84dd).
⚠️ Report is 276 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4631      +/-   ##
==========================================
- Coverage   99.21%   98.44%   -0.78%     
==========================================
  Files          42       48       +6     
  Lines        3182     3147      -35     
==========================================
- Hits         3157     3098      -59     
- Misses         17       41      +24     
  Partials        8        8              

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.43% <100.00%> (?)
-tags go_json	98.36% <100.00%> (?)
-tags nomsgpack	98.42% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.44% <100.00%> (?)
go-1.26	98.44% <100.00%> (?)
macos-latest	98.44% <100.00%> (-0.78%)	⬇️
ubuntu-latest	98.44% <100.00%> (-0.78%)	⬇️

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[Herrtian]

Added coverage for the existing directory path and the dir helper. Targeted upload-file tests pass locally.

[Herrtian]

Expanded the dir helper to avoid the remaining partial coverage line. Targeted upload-file tests pass locally.

[Herrtian]

Adjusted the directory setup path so the new helper branches are covered. Targeted upload-file tests pass locally with 100% coverage for the helper.