fix(markdown): rewrite stable file references in HTML srcset

[Valley-15]

Summary

When generating Markdown versions of published pages, stable /files/:id references inside raw HTML srcset attributes were not being rewritten to their resolved file URLs.

This caused image URLs such as:

to remain as /files/:id references in the generated Markdown instead of resolving to the actual file URL.

This change updates the existing HTML reference rewriting logic to also resolve srcset attributes using the same content-reference resolution already used for HTML src attributes.

Changes

• Rewrite stable /files/:id references in HTML srcset attributes.
• Keep the existing HTML src reference behavior unchanged.
• Add regression tests covering both HTML src and srcset file references.

Fixes #4593.

Validation

• bun test packages/gitbook/src/lib/markdownPage.test.ts
• bun run --cwd packages/gitbook unit
• bun run format:check
• bun run lint
• bun run --cwd packages/gitbook typecheck
• git diff --check

All checks pass.

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Latest commit: b44fcec

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