Elements: Guard against non-string className in render filter

[aaronrobertshaw]

This ticket tracks the backport of PHP files for the following Gutenberg update:

​https://github.com/WordPress/gutenberg/pull/78841

Adds a safeguard in elements block support rendering to prevent fatal errors when a block’s className attribute is not a string. The backport updates wp_render_elements_class_name() to bail early and return block content unchanged for invalid className values, and includes a PHPUnit test covering this regression case.

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.