Fix RM-15659

🧰 Changes

Fix callout rendering crash when the callout title contains variables — both legacy <> and MDX {user.name} syntax. In a recent change in #1362, the callout transformer serializes the title paragraph back to markdown via toMarkdown() to detect block-level syntax. When the title contains readme-variable or mdxTextExpression nodes, toMarkdown() throws "Cannot handle unknown node" because it has no handlers for these custom types. It turns out we needed to explicitly pass in the handlers for them for it to not crash & serialise them properly.

🧬 QA & Testing

> 🚧 Hello <<name>>                                                                                                                                
>                                                                                                                                                   
> Test                                                                                                                                             
                                                                                                                                                     
> 🚧 Hello {user.name}                                         
>> Test

• Verify both callouts render without console errors
• Verify the variable resolves in the title (shows the variable value, not raw syntax)
• Verify callout body ("Test") still renders