fix(utils): resolve site-relative paths in the eager Git VCS

[alexandrosang21]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes Sitemap <lastmod> is omitted with experimental_vcs: 'default-v2' in Docusaurus 3.10.2 #12322) and the maintainers have approved on my working plan.

Left unchecked deliberately: this is a small bug fix rather than a new API or substantial
change, so no working plan was agreed beforehand. It does close #12322.

Motivation

Fixes #12322. With future.experimental_vcs: 'default-v2', sitemap.xml is generated without
any <lastmod> values, while 'git-ad-hoc' and 'default-v1' produce them correctly for the
same site and Git history.

default-v2 selects the eager Git strategy in production. vcsGitEager keys its file map by
absolute paths:

// The Map keys should be absolute file paths, not relative Git paths
return [resolve(repoRoot, entry[0]), entry[1]];

but the lookup uses whatever path it is handed:

return init.filesMap.get(filePath) ?? null;

The sitemap plugin passes route.metadata.sourceFilePath, which RouteMetadata documents as
"expected to be relative to the site directory". So the lookup misses and returns null, and
getRouteLastmod drops <lastmod> for that route without any warning.

git-ad-hoc and default-v1 are unaffected because they shell out to git log -- <path>, which
resolves a relative path itself.

This resolves the path against the site dir when it is not already absolute. Absolute paths keep
working unchanged, so getFileCreationInfo and existing callers are unaffected.

Test Plan

Unit test. Added can read repo file info from a site-relative path to the existing
VSC Git Eager Strategy block. It fails on main and passes with this change. Every existing
eager test passes an absolute path via path.join(repoDir, ...), which is why this gap was not
covered.

pnpm vitest run packages/docusaurus-utils/src/vcs packages/docusaurus-plugin-sitemap
Test Files  5 passed (5)
     Tests  94 passed (94)

eslint, tsc --noEmit, oxfmt --list-different and cspell are all clean on the two changed
files.

End to end, using the reproduction repository from the issue
(eMUQI/docusaurus-vcs-sitemap-repro),
building the same site twice with only this patch differing:

@docusaurus/utils	<url>	<lastmod>
unmodified	1	0
with this change	1	1

Its npm run verify:repro asserts that default-v2 produces zero <lastmod>, so with the fix
applied that assertion now fails, which is the intended outcome.

Test links

Not applicable: this changes build-time sitemap metadata rather than anything rendered, so there
is no UI to preview.

Disclosure

This change was written with AI assistance (Claude), per the AI policy in CONTRIBUTING.md. I have
reviewed every line, verified the root cause against the reproduction repository above, and can
explain and defend the change in review.

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