feat(mdxish): always compile callouts to JSX in mdxishMdastToMd

[eaglethrost]

🎫 Resolve RM-16782

🎯 What does this PR do?

Updates the mdxish callout serializer (mdxishMdastToMd) to always compile callouts to <Callout> JSX syntax instead of the legacy MD blockquote format.

PR #17745 adds icon + color selection for callouts in the MdxishEditor UI, but the engine always serialized callouts to the legacy blockquote (> 📘 Hey), which dropped any custom icon/theme on round-trip. This PR persists those selections.

• Adds processor/transform/mdxish/callout-to-jsx.ts — a transform that converts every rdme-callout mdast node into an mdxJsxFlowElement (<Callout icon=… theme=…>), reusing toAttributes and the mdxJsxStringify extension already wired into mdxishMdastToMd.
• Registers the transform in mdxishMdastToMd (alongside mdxishTablesToJsx). The RMDX (mdx) pipeline and the shared processor/compile/callout.ts blockquote compiler are untouched, so RMDX behavior is unchanged.

🧪 QA tips

• The transform is the exact inverse of transformCallout in mdxish-jsx-to-mdast.ts: a body-only (empty) callout drops its placeholder title paragraph so re-parsing re-derives empty from the absence of a leading heading (no fragile empty attribute needed).
• Default-filling mirrors readme-to-mdx (icon→theme via themes, theme→icon via defaultIcons) so both attributes always persist.
• Children are mutated in place to preserve array identity, so nested callouts in the body are still visited and converted.
• Round-trips are stable, e.g. > 🚧 It works! → <Callout icon="🚧" theme="warn">### It works!…</Callout>, body-only callouts re-derive empty: true, and authored JSX callouts round-trip unchanged in shape.

📸 Screenshot or Loom

N/A — engine-side serialization change, covered by unit tests.