Port upstream code fence info string fix

[Renakoni]

Summary

• Port upstream MarkText PR #4846 into the Android Muya copy.
• Preserve full fenced code block info strings during markdown -> state -> markdown round trips.
• Add regression coverage for Pandoc/RMarkdown attributes, language attributes, plain languages, language-less fences, and stale info after language edits.

Upstream Source

• Upstream PR: fix(muya): preserve fenced code block info string on round-trip (#4770) marktext/marktext#4846
• Upstream issue: [Bug] Marktext removes attributes from code block marktext/marktext#4770

Android Relevance

This affects Android document saving. Android uses Muya's state import/export pipeline to edit and serialize markdown. Before this fix, fenced code block info strings were reduced to the first word when saved: for example ```js title="app.js" became ```js, and Pandoc/RMarkdown attribute blocks like ```{example, listing1-name} were truncated. That is data loss in Android-edited markdown files.

Change Details

• Keep meta.lang as the first info-string word for highlighting and language class behavior.
• Preserve the original full fenced info string additively in meta.info when it carries more than lang.
• Serialize meta.info only while its first word still matches meta.lang; if the language is later edited, the edited language wins and stale attributes are not re-emitted.
• Add codeFenceInfoString.spec.ts for the round-trip and stale-info cases.

Verification

• pnpm --dir third_party/muya exec vitest run src/state/__tests__/codeFenceInfoString.spec.ts src/state/__tests__/codeFenceLength.spec.ts passed.
• pnpm build passed.