What?

In the Template Part block, set the $template_part_file_path variable (for use by the render_block_core_template_part_file and render_block_core_template_part_none filters).

Why?

Fixes #56013. This was broken by #52892.

How?

By calling _get_block_template_file() (which duplicates some logic also found deeper in the call stack of get_block_file_template). But alas, the relevant information cannot be inferred from the WP_Block_Template object returned by get_block_file_template, nor does the eponymous filter expose any useful information.

Testing Instructions

Quoting #56013:

add_action( 'render_block_core_template_part_file', function( $template_part_id, $attributes, $template_part_file_path ) {
    var_dump( $template_part_file_path );
}, 10, 3 );

Load the front page of a vanilla WordPress site using the Twenty Twenty Four theme and observe that [...] the template part file path [is logged].

This [fix] can also be [tested] by installing the Query Monitor plugin and viewing the "Template" panel, which [should show] the path to each Template Part block that was loaded on the page.