Check postId returns a published wp_template_part (#26734)

[bobbingwide]

Description

This PR implements some additional tests when loading a template part by postId.
If the post type exists but it's not a wp_template_part or the post status is not publish then the template is not loaded.

The template loading logic is now slightly different from before.
If the postId is set and subsequently determined to be invalid then the template part is not loaded.
To help with problem determination the "Template Part Not Found" message now includes the slug.

How has this been tested?

Using Twenty Twenty-One Blocks I created a 404.html template file which contained.

<!-- wp:template-part {"slug":"issue-26734","theme":"twentytwentyone-blocks", "postId": 1 } /-->

I adjusted the postId attribute for each test, where the URL was for content that couldn't be found.

postId	Referenced post type	Referenced post status	Result
1	post - Hello World	n/a	Template Part Not Found: issue-26734
11	wp_template_part	draft	Template Part Not Found: issue-26374
25	wp_template_part	publish	Displayed the header template part.

This change prevents posts which are not of the correct post type and status from being loaded as template parts.
It doesn't deal with other problems with invalid attributes.

In the third test the postId loaded was a template part but it was not for the given slug and may not have been for the selected theme. This will have to be addressed once the requirements are better understood.

Screenshots

Types of changes

Fixes #26734

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[bobbingwide]

What do I need to do to progress this PR?

[youknowriad]

Hi @bobbingwide sorry this took so long. I didn't see the PR personally. The PR seems good but there seem to be a conflict now, would you mind rebasing it?

[bobbingwide]

I'll have a go if someone else doesn't get there first.

[david-szabo97]

Closing this. postId isn't an attribute of the Template Part block anymore. We are using the theme and slug attributes to fetch the correct template part.

[bobbingwide]

Oh good.