fix(mdx-loader): avoid transforming dotted directory links into asset…

[thanasis2028]

… fixes (#11940)

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 #0000) and the maintainers have approved on my working plan.

Motivation

This PR fixes a bug in markdown link asset detection for local links that point to existing directories whose names contain a dot.

Today, transformLinks uses path.extname() to decide whether a local link looks asset-like. Because of that, links such as ../Project.Tests are treated as file assets instead of directory links. When such a path exists on disk as a directory, Docusaurus rewrites it to a webpack require(), which later fails during the build with a Module not found error.

This pattern is common in ecosystems that use dotted directory names, such as .NET test projects (Project.Tests).
This PR updates transformLinks so that existing local directories are not transformed into asset requires.

Test Plan

I added a regression test in packages/docusaurus-mdx-loader/src/remark/transformLinks/__tests__/index.test.ts covering an existing dotted directory link.

I verified the change with:

corepack yarn test packages/docusaurus-mdx-loader/src/remark/transformLinks/__tests__/index.test.ts --runInBand
corepack yarn test packages/docusaurus-mdx-loader/src/remark/resolveMarkdownLinks/__tests__/index.test.ts --runInBand

Both test suites passed.

I also verified the behavior manually with a local reproduction using a markdown link like:

[Some.Directory](../Some.Directory)

pointing to an existing directory named Some.Directory.

Before this change, that kind of link could be turned into a webpack asset require and fail the build with Module not found.
After this change, Docusaurus no longer treats the existing dotted directory as an asset. The link remains a normal link and is handled by the regular broken-link pipeline instead of crashing during asset resolution.

Test links

No UI changes

Related issues/PRs

#11940

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