fix(session-ui): guard markdown preload to prevent renderer unresponsive

[xu-xiaofei]

Issue for this PR

Closes #39478

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fixes intermittent Desktop renderer unresponsiveness on Home page caused by expensive markdown preloading/parsing on the renderer main thread (preloadMarkdown -> marked lexer).

Main changes:

• add markdown parse guards (size / line-count / long-line thresholds)
• cap background preload budget
• fallback render for over-limit markdown blocks
• add regression tests and incident note

How did you verify your code works?

• packages/session-ui
  • bun run test passed (79/79)
  • bun typecheck passed
• local smoke checks
  • backend responds on :4096
  • app starts on :4444

Screenshots / recordings

N/A (stability fix, no UI layout change)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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