fix(mdxish): properly parse embed magic blocks

[eaglethrost]

	Fix RM-XYZ

🧰 Changes

There were 2 bugs regarding [block:embed] magic blocks in mdxish:

• It could crash the page
• Even if they don't it wasn't being parsed correctly
• Demo: https://github.com/user-attachments/assets/60f2de91-3b25-4fbb-a3e1-ec7d6de9803a

Fixes:

• When parsing embed magic blocks & setting the children, don't send in null as fallback, because it can create a type error later on. Use an empty string instead
• Create an embed handler to map the embed node magic blocks to the Embed component defined in markdown

Demo of it working now:

Screen.Recording.2026-01-06.at.4.33.35.pm.mov

🧬 QA & Testing

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

1. In the readme site, create a project document with an embed item (e.g YouTube)
2. Turn on mdxish for the project in god mode
3. View the doc with embed content, it shouldn't crash and render the same as MDX

Example embed:

[block:embed]
{
  "url": "https://www.youtube.com/watch?v=FVikHLyW500&list=RD3-9V38W00CM&index=4",
  "provider": "youtube.com",
  "href": "https://www.youtube.com/watch?v=FVikHLyW500&list=RD3-9V38W00CM&index=4",
  "typeOfEmbed": "youtube"
}
[/block]

[eaglethrost]

Some minor input formatting changes

[eaglethrost]

The problematic code, accessing on null's