feat: add featured image caption block

[chickenn00dle]

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:

Closes https://linear.app/a8c/issue/NPPD-1187/captioncredit-block-for-featured-images

This PR adds a featured image caption block when a block theme is active.

How to test the changes in this Pull Request:

1. Go to any post and add a featured image or featured image block to the post.
2. Add the featured image caption block to the page
3. Save and view the post on the frontend. Confirm the featured image caption appears as expected
4. Back in the editor select the featured image caption block and add custom text
5. Save. View the featured image meta by selecting 'Replace' and confirm the caption did not change.
6. View the post on the frontend. Confirm the featured image caption appears as expected
7. Go back to the editor and remove the featured image.
8. Confirm the featured image caption block displays placeholder text asking you to add a featured image.
9. Save and view the post on the frontend. Confirm the featured image caption block is not present.
10. In the editor, add the query loop block and add the featured image and featured image caption blocks and save.
11. Verify the query loop displays the featured image captions correctly in the frontend

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

Adds a new dynamic Gutenberg block to display a post’s featured image caption and (optionally) image credit when using a block theme.

Changes:

• Registers a new newspack/featured-image-caption block (editor + server render).
• Adds an editor UI for overriding the caption via a customCaption attribute.
• Wires the block into block-theme-only registration paths (JS + PHP).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/blocks/index.js	Registers the new block and gates it behind block-theme-only registration.
src/blocks/featured-image-caption/index.js	Block registration metadata/settings (title, icon, keywords, edit component).
src/blocks/featured-image-caption/edit.js	Editor implementation: pulls featured image + derives caption/credit placeholder; supports custom override.
src/blocks/featured-image-caption/style.scss	Adds editor styling for the “no featured image” placeholder state.
src/blocks/featured-image-caption/block.json	Defines block name, attributes, context usage, and supports settings.
src/blocks/featured-image-caption/class-featured-image-caption-block.php	Server-side render callback for frontend output (caption + credit, or custom override).
includes/class-blocks.php	Loads the new block PHP implementation when a block theme is active.

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[Copilot]

The placeholder shown when no featured image is set does not match the PR testing instructions (it doesn’t prompt the user to add a featured image). Consider updating this placeholder copy to explicitly instruct the user to add/set a featured image so the editor state is self-explanatory.

[Copilot]

This new block adds logic to derive caption/credit from the featured image entity (and format credits) but there are no accompanying unit tests. Similar blocks in this repo have Jest/Testing Library coverage; consider adding tests for “no featured image”, “caption only”, “credit only”, and “caption + org credit” to prevent regressions.