Duotone: Use HTML API class_list for duotone wrapper class handling (backport)

[jonsurrell]

Backport Gutenberg change ​https://github.com/WordPress/gutenberg/pull/79531

Updates WP_Duotone_Gutenberg::restore_image_outer_container() to use WP_HTML_Tag_Processor::class_list() instead of manually reading and splitting the class attribute.

Adds unit coverage for moving a generated wp-duotone* class from the inner image figure to the restored outer .wp-block-image wrapper in classic themes.

Follow-up to [58313].

See https://core.trac.wordpress.org/ticket/65466#comment:2.

The tag processor already provides class-specific APIs, which handle HTML class-list semantics more directly than custom string parsing.

Ports the matching Gutenberg change to Core: ​https://github.com/WordPress/gutenberg/pull/79531.

Follow-up to r58313.

Trac ticket: https://core.trac.wordpress.org/ticket/65576

## What?

Follow-up to ​https://github.com/WordPress/gutenberg/pull/59764.

Updates WP_Duotone_Gutenberg::restore_image_outer_container() to use WP_HTML_Tag_Processor::class_list() instead of manually reading and splitting the class attribute.

Adds unit coverage for moving a generated wp-duotone* class from the inner image figure to the restored outer .wp-block-image wrapper in classic themes.

## Why?

The tag processor already provides class-specific APIs, which handle HTML class-list semantics more directly than custom string parsing.

## How?

• Replaces explode( ' ', $tags->get_attribute( 'class' ) ) with $tags->class_list().
• Adds PHPUnit coverage for the classic-theme duotone wrapper behavior.
• Moves fixture theme setup/teardown into the test lifecycle, matching nearby theme-dependent tests.

## Testing Instructions

1. Run vendor/bin/phpunit --filter WP_Duotone_Gutenberg_Test.
2. Run vendor/bin/phpcs lib/class-wp-duotone-gutenberg.php phpunit/class-wp-duotone-test.php.

## Use of AI Tools

OpenAI Codex was used to assist with the code changes and PR description. The author is responsible for reviewing the final changes.

Trac ticket:

## Use of AI Tools