Fix tableprefix and cart mysql

modules/billing/test_db_connection.php

@@ -10,8 +10,8 @@
  * accessible in production.
  */
 
-// Include database configuration
-require_once(__DIR__ . '/includes/config.inc.php');
+// Include billing bootstrap (loads config and DB helper)
+require_once(__DIR__ . '/bootstrap.php');
 
 // Create database connection
 $db = mysqli_connect($db_host, $db_user, $db_pass, $db_name);