fix(collections): restore list items in posts with card indicator

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

In Collections, the 'Card' Collection Indicator Style used $block['innerHTML']; to grab each block in the content, which accidentally dropped inner blocks like <li> in List blocks, or columns in Columns blocks.

This hotfix switches it to use render_block( $block ); to preserve the inner blocks; it also adds a test for this specific case (making sure nested child blocks are preserved).

Closes NPPM-2831

How to test the changes in this Pull Request:

1. On a site running Collections, navigate to Newspack > Settings > Collections, and switch the 'Collection Indicator Style' to 'Card':

2. Create a new post with a List block and some list items in it. Publish.
3. View the post on the front-end; note that the list doesn't appear.
4. Inspect the post content using the element inspector - note that the empty <ul> tags still exist, just the list items do not.
5. Apply this PR.
6. Refresh your post; confirm that your list items appear.
7. Smoke test and confirm the post otherwise looks okay.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[Copilot]

Pull request overview

Hotfix for Collections "Card" indicator style that was stripping inner blocks (e.g. <li> items in list blocks) by using $block['innerHTML'] instead of render_block().

Changes:

• Switch reassembly in Content_Inserter::insert_after_nth_block from innerHTML to render_block() so inner/dynamic blocks render correctly.
• Add a regression test exercising a core/list with core/list-item children.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
includes/collections/class-content-inserter.php	Use render_block() to preserve inner-block content when reassembling parsed blocks.
tests/unit-tests/collections/class-test-content-inserter.php	Adds regression test verifying list items survive insertion.

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