Missing test cases for KaTeX dollar syntax and option gating in message-parser

[Khizarshah01]

Description:

The current katex.test.ts only has 2 test cases and only covers parenthesis syntax (\[...\] and \(...\)).

Missing test cases include:

• Dollar syntax ($$...$$) block KaTeX
• Dollar syntax ($...$) inline KaTeX
• Option gating — KaTeX delimiters treated as plain text when options are disabled

This weakens confidence in KaTeX parsing behavior, especially relevant ahead of the planned parser rewrite where test coverage is critical.

Steps to reproduce:

1. Open packages/message-parser/tests/katex.test.ts
2. Observe only 2 test cases exist
3. Dollar syntax and option gating have no test case

Expected behavior:

KaTeX parsing should have test coverage for all supported syntax variants and option flags.