fix: preserve asset URLs when trailingSlash is enabled

[Sudemc]

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: Not applicable; this is a scoped bug fix for asset links with trailing slash true can be broken #6282.

Motivation

Fixes #6282.

When trailingSlash: true is enabled, generated Markdown asset URLs are treated like internal routes by the Link component. This appends a trailing slash to URLs such as /assets/files/example.docx, causing the browser to request an invalid asset path.

The MDX transform already knows when a link resolves to a real asset file. This change carries that information to Link through an internal marker so only confirmed asset URLs skip route-specific trailing-slash normalization. Normal internal routes keep their existing behavior.

Summary

• mark resolved Markdown asset links so their exact URLs are preserved
• consume the internal marker in Link without forwarding it to the DOM
• leave trailing-slash handling unchanged for normal internal routes
• add regression coverage for both the MDX transform and Link behavior

Test plan

• pnpm test packages/docusaurus/src/client/exports/__tests__/Link.test.tsx packages/docusaurus-mdx-loader/src/remark/transformLinks/__tests__/index.test.ts
• pnpm --filter @docusaurus/core build
• pnpm --filter @docusaurus/mdx-loader build
• scoped ESLint and Oxfmt checks on all changed TypeScript files

All 38 targeted tests pass, and both affected packages build successfully with Node.js 24.14.

Test links

• Deploy preview: https://deploy-preview-12323--docusaurus-2.netlify.app/

Related issues/PRs

• Fixes asset links with trailing slash true can be broken #6282

This contribution was AI-assisted. The implementation and generated changes were manually reviewed and verified.

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