FSE: Strip post ids from template part blocks on export.

[fklein-lu]

This ensures that exported FSE themes work on other sites.

Description

Fixes the export of FSE themes so that they can be activated on other sites

How has this been tested?

Tested:

1. Automated via a WP unit test.
2. Manually by exporting a template with a template part on local, and verifying the output.

Types of changes

Bug fix: Fixes the issue outlined in #25845.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.

[skorasaurus]

Thanks for the PR.

I've tested it manually and functions as expected on my end; the postids are no longer included in the template-part.

(Made edits to the footer and header both through https://site.test/wp-admin/admin.php?page=gutenberg-edit-site and through https://site.test/wp-admin/edit.php?post_type=wp_template_part ; then exported the zip.

[youknowriad]

I'd love some thoughts about how to make this more generic across blocks. the same issue can happen for reusable blocks for instance and even more complex for image blocks (and other media blocks) where the id is in the markup.

[youknowriad]

Thinking more, I guess you could argue that the template parts are a bit special in the sense that they are actually already exported as files to the themes. So yeah, this might have more priority.

I didn't test but I'm on board with this

[youknowriad]

Let's get this in. Seems like a decent fix for now.