Elements: Align class name parsing with custom CSS implementation

[westonruter]

What?

Follow-up to #78841 and its core companion WordPress/wordpress-develop#12028 (committed in r62475), bringing the additional hardening developed during the core review back to Gutenberg so the two stay in sync.

During the core review of #78841, wp_render_elements_class_name() was aligned with wp_render_custom_css_class_name() (#78217 / r62359). This PR applies the same alignment here.

Why?

gutenberg_render_elements_class_name() matched the wp-elements-* class with \bwp-elements-\S+\b. Because a hyphen is a \b word boundary in regex, a class such as my-wp-elements-foo was erroneously matched. Parsing the className attribute into tokens per the HTML spec and checking each with str_starts_with() fixes this, and avoids the regex engine entirely for the common case via an upfront str_contains() short-circuit.

How?

• gutenberg_render_elements_class_name(): replaced the regex with a str_contains() guard followed by a strtok() walk over the class tokens; added an @phpstan-param array shape for $block.
• gutenberg_render_custom_css_class_name(): refactored to use a shared $class_name_prefix variable and $matched_class_name so both functions remain structurally identical.
• Added a regression test for the my-wp-elements-foo substring-prefix case (the non-string className test already existed).

These match the corresponding core changes in r62475 byte-for-byte (modulo the gutenberg_/wp_ prefixes).

Testing Instructions

• Run the elements block support PHP tests: npm run test:unit:php -- --filter WP_Block_Supports_Elements_Test
• Verify test_elements_block_support_class_with_invalid_elements_prefix and test_elements_block_support_class_with_non_string_class_name pass, and existing tests continue to pass.

🤖 Generated with Claude Code

[github-actions]

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>

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[github-actions]

Flaky tests detected in b4ec983.
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/27177955117
📝 Reported issues:

• [Flaky Test] Should save the changes #78689 in /test/e2e/specs/editor/plugins/wp-editor-meta-box.spec.js
• [Flaky Test] shouldn't conflict with server-side autosave #78706 in /test/e2e/specs/editor/various/autosave.spec.js

[aaronrobertshaw]

Thanks for the PR and the core tweaks @westonruter 🙇

✅ Changes here faithfully match related core commits
✅ Backport changelog looks good
✅ New and existing tests pass

LGTM 🚢