Block editor: fallback for unsynced pattern insertion - #81323

What?

Closes: #81041

Fixes the insertion of unsynced patterns when their content cannot be parsed into blocks.

Unsynced patterns now:

• Insert their parsed blocks directly when parsed content is available.
• Fall back to the core/block reference when an unsynced reusable pattern has no parsed blocks.
• Preserve the original reusable pattern behavior for core/block items with a valid ref.
• Avoid incorrectly wrapping child-only blocks, such as core/button, in a new core/buttons block.
• Remain available in the pattern inserter when their root block has a single allowed parent block.

Why?

Unsynced patterns can contain malformed or incomplete block content. In these cases, the parser may return no blocks, causing the inserter to insert nothing.

How?

The unsynced pattern insertion paths now use the following behavior:

1. For a core/block item with a valid ref, insert the reusable pattern block directly.
2. For other unsynced items with parsed content, insert cloned or parsed blocks directly.
3. When no parsed blocks are available, insert a fallback block using the original block name and attributes.
4. Allow patterns containing a child-only root block to appear when its single parent block is allowed at the insertion location.
5. Do not synthesize a parent block around the parsed unsynced content.

Testing Instructions

1. Open a post or page in the block editor.
2. Create or locate an unsynced pattern containing editable blocks.

As per original issue #81041

<!-- wp:button {"className":"ewww-button aligncenter"} -->
<div class="wp-block-button ewww-button aligncenter"><a class="wp-block-button__link wp-element-button" href="https://ewww.io/fake-link/" rel="">Fake Text!</a></div>
<!-- /wp:button -->

3. Open the inserter and select the unsynced pattern.
4. Confirm that the pattern content is inserted directly and remains editable.

Before ( Unable to add unsynced pattern )	After ( Able to add unsynced pattern )

Use of AI Tools

Used the help of AI tool like GPT 5.5 for code review.