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/AfterSale.php

@@ -137,7 +137,7 @@ class AfterSale {
         $items = $wpdb->get_results($wpdb->prepare($sql, $params), ARRAY_A);
         $items = array_map([$this, 'decorate_record'], (array) $items);
 
-        if ($include_order_context && !empty($items) && class_exists('OrderRepository')) {
+        if ($include_order_context && !empty($items) && class_exists('AKSRT\\Payment\\OrderRepository')) {
             $contexts = OrderRepository::get_instance()->get_admin_order_context_map(array_column($items, 'order_number'));
             foreach ($items as &$item) {
                 $item['order_context'] = $contexts[(string) ($item['order_number'] ?? '')] ?? [];
@@ -174,7 +174,7 @@ class AfterSale {
             return null;
         }
 
-        $record['order_context'] = class_exists('OrderRepository')
+        $record['order_context'] = class_exists('AKSRT\\Payment\\OrderRepository')
             ? OrderRepository::get_instance()->get_admin_order_context((string) ($record['order_number'] ?? ''))
             : [];
 
@@ -561,7 +561,7 @@ class AfterSale {
     }
 
     private function sync_order_status(array $record, string $new_status, string $note = ''): void {
-        if (!class_exists('OrderRepository') || !class_exists('Order')) {
+        if (!class_exists('AKSRT\\Payment\\OrderRepository') || !class_exists('AKSRT\\Payment\\Order')) {
             return;
         }