Fix Template Part auto-drafting to support sub-directories. (#25063)

* refactor for nested files

* merge arrays outside of function

* lint wants function doc

lib/template-parts.php

@@ -211,13 +211,28 @@ function filter_rest_wp_template_part_query( $args, $request ) {
 
 		// Ensure auto-drafts of all theme supplied template parts are created.
 		if ( wp_get_theme()->get( 'TextDomain' ) === $request['theme'] ) {
+			/**
+			 * Finds all nested template part file paths in a theme's directory.
+			 *
+			 * @param string $base_directory The theme's file path.
+			 * @return array $path_list A list of paths to all template part files.
+			 */
+			function get_template_part_paths( $base_directory ) {
+				$path_list = array();
+				if ( file_exists( $base_directory . '/block-template-parts' ) ) {
+					$nested_files      = new RecursiveIteratorIterator( new RecursiveDirectoryIterator( $base_directory . '/block-template-parts' ) );
+					$nested_html_files = new RegexIterator( $nested_files, '/^.+\.html$/i', RecursiveRegexIterator::GET_MATCH );
+					foreach ( $nested_html_files as $path => $file ) {
+						$path_list[] = $path;
+					}
+				}
+				return $path_list;
+			}
+
 			// Get file paths for all theme supplied template parts.
-			$template_part_files = glob( get_stylesheet_directory() . '/block-template-parts/*.html' );
-			$template_part_files = is_array( $template_part_files ) ? $template_part_files : array();
+			$template_part_files = get_template_part_paths( get_stylesheet_directory() );
 			if ( is_child_theme() ) {
-				$child_template_part_files = glob( get_template_directory() . '/block-template-parts/*.html' );
-				$child_template_part_files = is_array( $child_template_part_files ) ? $child_template_part_files : array();
-				$template_part_files       = array_merge( $template_part_files, $child_template_part_files );
+				$template_part_files = array_merge( $template_part_files, get_template_part_paths( get_template_directory() ) );
 			}
 			// Build and save each template part.
 			foreach ( $template_part_files as $template_part_file ) {