RTC: Fix CRDT serialization of nested RichText attributes - #76597

What?

Closes #76555.

Fixes table block cell content syncing (and any other nested array-type attributes with nested rich text) in real-time collaboration.

Why?

The corruption happens at Yjs encode time, when the CRDT doc is synced or persisted:

1. Page loads and blocks are parsed from post content. Table cell content becomes RichTextData class instances, but deeply nested (e.g. body[].cells[].content).
2. makeBlocksSerializable runs but only converts top-level RichTextData to strings. The nested instances of RichTextData inside the table's body/head/foot aren't properly mapped to strings with .valueOf() and instead stay as deeply-nested objects instead of being properly converted to serializable data.
3. When Yjs encodes the doc, it serializes these RichTextData instances the same way it would any plain object, using Object.keys(). Since RichTextData stores its data in a private field, there are no enumerable keys, so each cell's content is encoded as {}.
4. This is stored in post meta and sent to peers. On reload, the persisted doc is decoded and the cells are shown as empty.

How?

Added a serializeAttributeValue helper that recursively walks attribute values and converts any deeply-nested RichTextData instances to their HTML string representation via .valueOf().

This approach handles the table block's deeply nested structure (body[].cells[].content) and will also cover any other blocks that store RichTextData inside array or object attributes.

Testing Instructions

Here's the original reproduction from this comment:

1. Enable RTC via Settings -> Writing -> "Enable real-time collaboration" checkbox.

2. Create a post with a table:

   npm run wp-env run cli -- wp post create --post_title='Table Test' --post_status='publish' --post_content='<!-- wp:table --><figure class="wp-block-table"><table class="has-fixed-layout"><tbody><tr><td>1</td><td>2</td></tr><tr><td>3</td><td>4</td></tr></tbody></table></figure><!-- /wp:table -->'

3. As user A, load the post in one tab with RTC enabled.

4. As user B, load the same post in a second tab with RTC enabled.

5. See user A's table get cleared out.

   Screen.Recording.2026-03-17.at.11.47.56.AM.mov

On this branch (fix/crdt-table-emptying), you should no longer see the table cleared.

Also see the added unit tests:

npm run test:unit -- --testPathPattern='core-data/src/utils/test/crdt-blocks' --testNamePattern='table block'

which failed before these changes.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Used for: Identifying the root cause and implementing the fix.