Add a mechanism for themes to prevent insertion of new blocks at root of block template part

[fabiankaegy]

What problem does this address?

This came up while testing #42729. When a theme provides a block template part there are cases where the theme wants to wrap a template part in a single element and only allow the insertion of new blocks within that root. Let's take a site-wide notification area or even a Cookie banner as an example. As a theme author, I want editors to be able to visually edit the content of that template part. But I want to ensure that all the content is located within a full-width aligned group block.

I can already use the block locking attributes to prevent editors from being able to remove the wrapping group block for example. But I currently cannot prevent editors from inserting any additional blocks at the root level of the template part. Ideally, there would be a way similar to how the templateLock on custom post types works.

What is your proposed solution?

It would be great to be able to define a templateLock at the level of individual template parts. This could be achieved via a metadata header in the .html template part file. Or via the templateParts key in theme.json.

[mediaformat]

I just ran into this issue in a hybrid theme, the template part in question is called via block_template_part( 'example-block' ). In this case the proposed solutions (metadata header) would work.