[ZAMB-257] Add MathJax rich-text support

[ocorona92]

Summary

• add an official MathJax plugin for the rich-text editor
• preserve inline and display delimiters across Markdown and Rich Text modes
• support live typing, Markdown paste, escaped dollars, and code spans
• delegate typesetting and cleanup to host-provided callbacks

Context

Enterprise editors do not have access to the optional Markdown preview mode, so MathJax needs to render directly in Rich Text. The StackOverflow integration will be submitted separately after this editor API is reviewed.

Validation

• npm run test:unit -- --runInBand --testPathIgnorePatterns=/node_modules/ \\.e2e\\.test /dist/ — 51 suites passed, 1,130 tests passed
• npm run lint — passed with two unrelated existing test warnings
• npm run prepublishOnly — Webpack and TypeScript passed
• manually verified typing, multiline paste, escaped dollars, code spans, mode round-tripping, and persistence on stackenterprise.local with MathJax 3.2.2

[changeset-bot]

🦋 Changeset detected

Latest commit: 08d5d64

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@stackoverflow/stacks-editor	Minor

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