Port upstream Mermaid export resilience fix

[Renakoni]

Summary

• Port upstream MarkText PR #4840 into the Android Muya copy.
• Isolate Mermaid export rendering failures per diagram instead of letting one invalid Mermaid block abort the whole HTML export path.
• Add a regression test for invalid Mermaid export resilience.

Upstream Source

• Upstream PR: fix(muya): isolate mermaid render failures during export (#4812) marktext/marktext#4840
• Upstream issue: [Bug] Failed to export document marktext/marktext#4812

Android Relevance

The Android repo carries Muya's MarkdownToHtml export implementation and its export tests. This bug is in that shared Muya path: MarkdownToHtml._renderMermaid() batched all Mermaid nodes into one mermaid.run() call, so a syntax error in one diagram rejected the entire render operation. Even though Android's current user-facing export surface is limited, keeping the embedded Muya export path correct avoids shipping a known upstream Muya failure in the codebase.

Change Details

• Convert Mermaid export rendering from one batched mermaid.run({ nodes }) call to one isolated run per Mermaid node.
• Catch per-diagram Mermaid failures and replace only the failed diagram with < Invalid Diagram >, matching the existing non-Mermaid diagram export fallback.
• Skip loading the Mermaid renderer when the document contains no Mermaid blocks.
• Add mermaidExportResilience.spec.ts covering:
  • invalid Mermaid does not reject renderHtml();
  • one invalid Mermaid block does not stop a later valid Mermaid block;
  • no-Mermaid documents do not load/run Mermaid.

Verification

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