Add KaTeX math rendering for LaTeX in responses

[rdwj]

Render `\(...\)` and `\[...\]` LaTeX via KaTeX. Uses `katex.renderToString()` during content processing to avoid CSS conflicts.

Consider making this opt-in or lazy-loading on first math detection to avoid overhead for non-math agents.

Reference: tool-calling-demo
See UPSTREAM_PORT.md section 1.5.