fix: defer streaming math rendering

[YishenTu]

Summary

• Fixes Feature request: Defer math rendering during streaming #597 by deferring dollar-delimited math parsing during live streaming renders.
• Adds a shared markdown utility that escapes $ and $$ delimiters outside inline code, fenced code, and raw HTML tags for transient streaming output.
• Re-renders completed text blocks once with the original markdown so final MathJax rendering remains unchanged.
• Adds a default-on general setting for deferring math rendering during streaming, including localized settings labels.

Validation

• node scripts/run-jest.js --config .context/jest-gh-597.config.cjs --runInBand --silent=false
• npm run typecheck
• npm run lint
• npm run test
• npm run build

The GH-597 validation script now reports zero live unescaped math renders and zero live unclosed math renders, while preserving one final render with completed markdown.