Rich text preview omits block images nested in list items

## Describe the bug

Decap CMS 3.15.0 preserves block images nested in list items in the rich text editor after #7896, but the preview pane omits those images. The saved Markdown remains correct; this is a separate preview rendering issue.

## To reproduce

Use a rich text field containing:

```markdown
1. First step.
2. Last step.

```

The editor displays the image under the second list item, but the preview shows only the list text.

## Expected behavior

The preview renders the image under the second list item.

## Root cause

`remarkRehypeShortcodes` only transforms shortcodes among the root node children. The built-in image component is represented as a shortcode, so an image nested under `list > listItem` reaches `remark-rehype` untransformed and is dropped from the preview output.

## Version

Decap CMS 3.15.0

Start by locating `remarkRehypeShortcodes` in the rich text preview pipeline and trace how `remark-rehype` handles nodes nested under `list > listItem`. Verify the reproduction with a block image inside a list item; done means the preview renders that image while saved Markdown remains unchanged.

Written by the indexing model from the issue text.