fix(file-backend): return correct last modified in head object

[ferhatelmas]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

last modified is wrong for head object in file backend

What is the new behavior?

last modified is correct as in get object

Additional context

closes #858

simplify date construction as well

[coderabbitai]

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • lastModified now consistently reflects the file system's actual modification time (mtime) across all object metadata responses.

• Tests

  • Added tests that verify lastModified is derived from the file's mtime and returned correctly for both head and get object metadata queries.

Walkthrough

Replaced internal lastModified computation in the file backend to use the filesystem modification time (data.mtime) consistently for getObject, headObject, and all related metadata responses (200/304 and range responses). Added tests that set a file's mtime and assert that headObject(...).lastModified and getObject(...).metadata.lastModified match the filesystem mtime. No public interfaces, eTag logic, or response structures were changed.

Assessment against linked issues

Objective	Addressed	Explanation
Ensure headObject returns the correct last-modified (matches GET/read) so HEAD and GET are consistent [#858]	✅

🚥 Pre-merge checks | ✅ 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: fixing the last modified value returned in HEAD object requests for the file backend.
Linked Issues check	✅ Passed	The PR successfully addresses issue #858 by correcting the lastModified source from a placeholder to the actual file mtime, ensuring HEAD and GET requests return consistent values.
Out of Scope Changes check	✅ Passed	All changes are within scope: file.ts modifications fix the lastModified logic, and test additions validate the fix against the stated requirement.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into master

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[coderabbitai]

🧹 Nitpick comments (1)

src/test/file-backend.test.ts (1)

191-215: Use withOptionalVersion to avoid hardcoding the file layout.

This keeps the test resilient if the versioning path format changes.

♻️ Proposed change

-    const filePath = path.join(tmpDir, `${bucket}/${key}/${version}`)
+    const filePath = path.join(tmpDir, withOptionalVersion(`${bucket}/${key}`, version))

[coveralls]

Pull Request Test Coverage Report for Build 22154238163

Details

• 4 of 6 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 76.263%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/storage/backend/file.ts	4	6	66.67%

Totals
Change from base Build 22154212801:	0.3%
Covered Lines:	26286
Relevant Lines:	34161

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💛 - Coveralls