Markdown incomplete-MDX fallback should append after void blocks

Problem

markdownToSlateNodesSafely is the recovery path when full markdown deserialization fails and the input ends with incomplete MDX.

That fallback already handled two sane cases:

• no complete blocks yet: return a new paragraph with the inline fallback text
• last complete block is non-void: append the fallback inline nodes to that block

But the void-block case was wrong. If the complete prefix ended in a void element like a horizontal rule, the function returned only the fallback paragraph and threw away the already-parsed nodes.

Root cause

The void-node branch returned:

return [newBlock];

That replaced the full completeNodes array instead of preserving it.

Fix

Append the fallback paragraph after the complete nodes:

if (ElementApi.isElement(lastBlock) && editor.api.isVoid(lastBlock)) {
  return [...completeNodes, newBlock];
}

Now the safe fallback keeps the parsed prefix and still exposes the incomplete MDX tail as literal text.

Verification

These checks passed:

bun test packages/markdown/src/lib/deserializer/utils/markdownToSlateNodesSafely.spec.tsx
bun test packages/markdown/src
bun run test:slowest -- --top 15 packages/markdown/src

The package build lane was blocked by an external tooling issue unrelated to this fix:

TypeError [ERR_INVALID_ARG_VALUE]: ... node:util.styleText ... Received [ 'underline', 'gray' ]

Prevention

When a fallback path combines “complete parsed output” with “literal recovery output,” add one test for each final-block shape:

• empty result
• non-void last block
• void last block

That catches the easy mistake where the recovery branch accidentally replaces the result instead of extending it.