fix(context): skip chmod on existing dir in SaveUploadedFile

[barry3406]

Pull Request Checklist

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• Open your pull request against the master branch.
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Description

Since v1.12.0, SaveUploadedFile calls os.Chmod on the destination's parent directory unconditionally after os.MkdirAll, which breaks saving into pre-existing directories the process does not own (for example SaveUploadedFile(file, "/tmp/foo.txt") fails with chmod /tmp: operation not permitted).

The fix stats the directory before MkdirAll and only chmods when the stat returned os.ErrNotExist, so the configurable mode from #4088 still applies to directories SaveUploadedFile actually creates while existing directories are left alone.

Added TestSaveUploadedFilePreservesExistingDirPermissions, which creates a temp dir with mode 0o700, saves into it with a different requested mode, and asserts the dir's mode survives and the file contents are correct. Added TestSaveUploadedFileAppliesModeToCreatedDir as a companion that saves into a nested directory which does not yet exist and asserts the new directory ends up with the requested mode.

Fixes #4622

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.37%. Comparing base (3dc1cd6) to head (6732132).
⚠️ Report is 275 commits behind head on master.

Files with missing lines	Patch %	Lines
context.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4629      +/-   ##
==========================================
- Coverage   99.21%   98.37%   -0.84%     
==========================================
  Files          42       48       +6     
  Lines        3182     3140      -42     
==========================================
- Hits         3157     3089      -68     
- Misses         17       42      +25     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.36% <50.00%> (?)
-tags go_json	98.29% <50.00%> (?)
-tags nomsgpack	98.35% <50.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.37% <50.00%> (?)
go-1.26	98.37% <50.00%> (?)
macos-latest	98.37% <50.00%> (-0.84%)	⬇️
ubuntu-latest	98.37% <50.00%> (-0.84%)	⬇️

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