fix: disable single-dollar math to prevent currency symbol text corruption

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Related GitHub Issue

Closes: #11648

Description

This PR attempts to address Issue #11648 where currency symbols (R$, US$, $100, etc.) cause text corruption in the markdown renderer. The remark-math plugin was treating single $ as inline math delimiters, causing text between dollar signs to be parsed as LaTeX.

The fix sets singleDollarTextMath: false in the remarkMath plugin config, as suggested by @diovannesn in the issue discussion. This is a 1-line config change that:

• Prevents single $ from being treated as math delimiters
• Preserves $$...$$ syntax for both inline and display math
• Zero preprocessing overhead

The trade-off is that inline math now requires $$x$$ instead of $x$, but $$x$$ renders as inline (not block) math, so functionality is preserved.

Test Procedure

• Added 3 new test cases to MarkdownBlock.spec.tsx:
  1. Currency symbols (R$100, US$50) render without corruption
  2. Double-dollar math expressions still render as KaTeX
  3. Single dollar signs are not treated as math delimiters
• All 7 tests pass: cd webview-ui && npx vitest run src/components/common/__tests__/MarkdownBlock.spec.tsx
• Lint and type checks pass across all packages

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue.
• Scope: Changes are focused on the linked issue (one config change + tests).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New tests have been added to cover the changes.
• Documentation Impact: No documentation updates required.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

Feedback and guidance are welcome.

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