fix(i18n): add translators comment for sprintf placeholder in rest-api

WordPress PHPCS requires a translators comment before sprintf() calls
that wrap __() with %s placeholders.

Agent-Session: claude/adopt-ci-20260616

includes/class-peptide-news-rest-api.php

@@ -104,6 +104,7 @@ class Peptide_News_Rest_API {
 		if ( ! preg_match( '/^\\d{4}-\\d{2}-\\d{2}$/', $value ) ) {
 			return new WP_Error(
 				'rest_invalid_date',
+				/* translators: %s: REST API parameter name */
 				sprintf( __( 'Invalid date format for %s. Expected Y-m-d.', 'peptide-news' ), $param ),
 				array( 'status' => 400 )
 			);