Forms: fix attribute appender

[CGastrell]

Proposed changes:

This PR is an optimization of how we inject template and widget attributes for further processing and consideration.

Related to #45997

As Claude puts it:

• Using parse_blocks() + serialize_blocks() is more robust than regex - WordPress automatically fixes many malformed JSON issues
• The refactored code gracefully handles edge cases that the regex approach would struggle with
• WordPress's block parser is very forgiving, which means our refactored method will work even with slightly malformed block markup

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1763554432894829-slack-CDLH4C1UZ

Does this pull request change what data or activity we track or use?

No

Testing instructions:

Run the tests with jetpack test php packages/forms --verbose.

Look at the code change and the test cases to verify those make sense.

For real world tests, the process is in charge of adding extra attributes to forms that have been placed on widgets and templates. This means that if you log the attributes of a form block nothing should change unless the form is on one of those entities.

// class-util.php, around line 420 where the method has found a jetpack/contac-form block
// log attrs before and after
	error_log( print_r( $block['attrs'], true ) );
	// Merge new attributes with existing ones.
	$block['attrs'] = array_merge(
		isset( $block['attrs'] ) ? $block['attrs'] : array(),
		$new_attr
	);
	error_log( print_r( $block['attrs'], true ) );

You can test those by switching to a theme that supports widgets, add a form to a widget and visiting on the frontend. Before the method is ran you should see the stored form attributes, after you should see an extra attribute, like block_template or widget among the form attributes.

[19-Nov-2025 16:50:02 UTC] Array
(
    [confirmationType] => text
    [jetpackCRM] =>
    [salesforceData] => Array
        (
            [organizationId] =>
            [sendToSalesforce] =>
        )

    [mailpoet] => Array
        (
            [listId] =>
            [listName] =>
            [enabledForForm] =>
        )

    [block_template] => canvas // <------- form block is on a template
)

cc @simison

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the fix/forms-attribute-appender branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/forms-attribute-appender

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Review, ...).
  
• ✅ Add a "[Type]" label (Bug, Enhancement, Janitorial, Task).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

---

Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Make sure to test your changes on all platforms that it applies to. You're responsible for the quality of the code you ship.
3. You can use GitHub's Reviewers functionality to request a review.
4. When it's reviewed and merged, you will be pinged in Slack to deploy the changes to WordPress.com simple once the build is done.

If you have questions about anything, reach out in #jetpack-developers for guidance!

---

Jetpack plugin:

The Jetpack plugin has different release cadences depending on the platform:

• WordPress.com Simple releases happen as soon as you deploy your changes after merging this PR (PCYsg-Jjm-p2).
• WoA releases happen weekly.
• Releases to self-hosted sites happen monthly:
  • Scheduled release: December 2, 2025

If you have any questions about the release process, please ask in the #jetpack-releases channel on Slack.

[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/contact-form/class-util.php	32/176 (18.18%)	-3.01%	-1 💚

Full summary · PHP report · JS report

_{Coverage check overridden by Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR .}