fix(ns): class_exists() must use FQCN after PSR-4

Bare class_exists('Balance') looks up global Balance, always false
under AKSRT\* namespaces → REST 403 on balance exchange and friends.
Rewrite string checks to AKSRT\{Domain}\{Class}.

src/AKSRT/Payment/ShippingPart1.php

@@ -10,7 +10,7 @@ defined('ABSPATH') || exit;
 trait ShippingPart1 {
 
     private function send_shipping_notification(string $order_number, string $carrier, string $tracking_no, string $note = ''): void {
-        if (!class_exists('OrderRepository')) {
+        if (!class_exists('AKSRT\\Payment\\OrderRepository')) {
             return;
         }
 
@@ -117,7 +117,7 @@ trait ShippingPart1 {
     }
 
     private function sync_order_status(string $order_number, string $shipping_status, string $note = ''): void {
-        if ($order_number === '' || !class_exists('OrderRepository') || !class_exists('Order')) {
+        if ($order_number === '' || !class_exists('AKSRT\\Payment\\OrderRepository') || !class_exists('AKSRT\\Payment\\Order')) {
             return;
         }