fix(frontend): preserve code during Markdown table normalization

[cangen185-sudo]

Description

The document chunk preview runs normalizeSpuriousTablePrefixes() before marked.parse(). It currently repairs pipe-delimited rows even inside literal Markdown code blocks. A fenced table example gains an extra separator row; examples of MarkItDown's empty-prefix output lose their first two rows. Four-space/tab-indented code can also be changed.

Limit table repair to non-code rows and track backtick/tilde fences, including the same-character/minimum-length closing rule, indentation, unterminated fences and backticks in info strings. No dependencies or public APIs change. Ordinary MarkItDown tables keep their existing repair behavior.

Type of Change

• Bug fix
• Test

Related Issue

New locally reproduced defect. This does not address #3962; that single-tilde rendering issue already has #3963.

Testing

Baseline: bccb4b151bae403508da77fbb174efc79dc47c1a (main). Windows, Node.js 24.16.0.

• The same 10 added regressions on unchanged source: 8 failed, 2 passed.
• With the fix: 10 passed.
• npm test -- src/utils/markdownTableNormalize.test.ts src/utils/documentPreviewMarkdown.test.ts src/utils/chatMarkdownRenderer.test.ts: 59 passed.
• npm ci --ignore-scripts --no-audit --no-fund: passed; package-lock unchanged.
• npm run type-check: passed.
• git diff --check: passed.

No live backend/model or full-repository suite was run. The tests cover this preprocessing function and adjacent Markdown renderers. This is a targeted fence guard, not a full Markdown parser or an overhaul of table recognition.

Checklist

• Diff has no whitespace errors
• Changed source follows the existing formatting
• Targeted tests pass
• Full-repository checks not run; scope documented above
• Added regression tests
• Updated the function's documentation
• No breaking public API change
• Contributor's human review

Screenshots / Recordings

The defect can be reproduced without a server using this input:

```markdown
| A | B |
| x | y |
```

Before: the rendered code gains | --- | --- | between its two rows. After: both literal rows are unchanged. The dedicated tests assert exact source preservation. No CSS/layout changes are made.

AI assistance

Codex assisted with source audit, reproduction, implementation and local tests. This PR is submitted as a draft for contributor review; no human review or maintainer acceptance is claimed.