Replace regex with tag processor for duotone class render

[ajlende]

What?

Updates duotone to use WP_HTML_Tag_Processor instead of preg_replace.

Why?

WP_HTML_Tag_Processor is a more robust solution for adding a class in PHP.

Fixes #49169

How?

Replaces regex code with WP_HTML_Tag_Processor for adding the duotone filter class.

Testing Instructions

1. Open a post with duotone filters applied.
2. See that the filters still render.

[github-actions]

Flaky tests detected in 9c30a2c.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4472838415
📝 Reported issues:

• [Flaky Test] allows tabbing to the inspector from the last block #48504 in /test/e2e/specs/site-editor/writing-flow.spec.js

[getdave]

I tested this with some posts with Duotone applied and it seemed to work. Code looks good based on what I know about this class.

[scruffian]

LGTM

[jeryj]

It looks like add_class gets a free pass and the if statement isn't necessary/desired here.

Suggested change

	if ( $tags->next_tag() ) {
	$tags->add_class( $filter_id );
	}
	$tags->add_class( $filter_id );

[ajlende]

The $tags->next_tag() returns a boolean if it finds a tag or not, so the if statement is checking to make sure that we have an element to add the class to, not that the class already exists on the element. The only reason I think it might return false is if you have an empty string or malformed HTML.

For reference, I based this off of how it was used in #46625.