Elements: Guard against non-string className in render filter

[aaronrobertshaw]

Trac ticket: https://core.trac.wordpress.org/ticket/65379

This PR brings the changes from the following Gutenberg PR to core:

WordPress/gutenberg#78841

Description

Hardens the elements block support render filter against invalid block attribute data.

wp_render_elements_class_name() currently reads attrs.className and passes it into preg_match(). While className is expected to be a string, malformed or corrupted stored block data can contain other types (for example, an array), which can trigger a fatal TypeError in PHP 8+.

This backport adds a defensive type check so that when className is not a string, the function returns the original block content unchanged instead of attempting regex matching.

A regression test is also added to ensure non-string className values do not cause fatals and continue to fail gracefully.

Testing

• Run the PHPUnit tests for elements block support: wpRenderElementsSupport.php
• Verify the new regression test passes: test_elements_block_support_class_with_non_string_class_name
• Verify existing tests in the same file continue to pass.

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[andrewserong]

This defensive check makes good sense to me, the added test looks good and is passing for me locally.

LGTM 🚀

[mukeshpanchal27]

Suggested change

	* @ticket 65379
	*
	* Block attributes such as `className` are always expected to be strings,
	* however invalid stored data can result in other types being present. The
	* render filter should fail gracefully rather than passing an array to
	* `preg_match()`.
	* Block attributes such as `className` are always expected to be strings,
	* however invalid stored data can result in other types being present. The
	* render filter should fail gracefully rather than passing an array to
	* `preg_match()`.
	*
	* @ticket 65379

Nit-pick