[404kernel]

Motivation

Fixes #12476.

When trailingSlash: true and baseUrl !== '/', running docusaurus serve returned 404 for certain static assets.
Specifically:

1. looksLikeAsset = !!req.url.match(/\.[a-z\d]{1,4}$/i); failed on longer extensions such as .woff2 (5 characters) or .webmanifest (11 characters).
2. Any asset URL containing query parameters or hash fragments (e.g. /assets/styles.css?v=1.0) failed the regex match.
3. When looksLikeAsset returned false, applyTrailingSlash appended a trailing slash (e.g. /myBase/font.woff2/), causing serve-handler to attempt resolving index files and returning 404.

Solution

• Replaced the character-length regex check with a robust isAssetUrl(url, options) helper:
  • Strips query parameters and hash fragments before evaluation.
  • Matches standard file extensions without arbitrary length limits.
  • Verifies whether the target path matches a directory or file in outDir when available.
• Added comprehensive unit tests in packages/docusaurus/src/commands/__tests__/serve.test.ts covering root baseUrl, custom baseUrls, font assets (.woff2), manifest assets, and query strings.

Test Plan

• Ran vitest packages/docusaurus/src/commands/__tests__/serve.test.ts (all 7 tests passing).
• Verified formatting with oxfmt.

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