fix: allow parsing empty callouts like > without breaking

[maximilianfalco]

	Fix CX-2520

🧰 Changes

Adds a format option to calloutTransformer in both mdx and mdxish to control empty blockquote handling:

• when format === 'md', empty blockquotes are replaced with paragraphs containing the stringified content ('>')
• when format === 'mdx' (or undefined) they remain as blockquotes to allow rendering as empty callouts.

This fixes errors when processing standalone '>' in MD format and preserves empty callout rendering in MDX. Includes tests for both formats in the transformer, MDX serialization, and mdxish processing pipelines.

The main reason the logic is split between these formats is because the excerpt component in the page headers uses the MD format while the body uses the MDX format. This done to allow cases like this:

🧬 QA & Testing

• Broken on production.
• Working in this PR app.