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/ChargeCard.php

@@ -330,7 +330,7 @@ class ChargeCard {
 
         if (!$this->mark_used_cas($card_id, $user_id, $order_number)) {
             // Race lost.
-            if (class_exists('Order')) {
+            if (class_exists('AKSRT\\Payment\\Order')) {
                 Order::get_instance()->update_status($order_number, Order::STATUS_CANCELLED, [
                     'note' => __('卡密已被使用，订单取消', 'aksrt'),
                 ]);
@@ -343,7 +343,7 @@ class ChargeCard {
             return $fulfill;
         }
 
-        if (class_exists('Order')) {
+        if (class_exists('AKSRT\\Payment\\Order')) {
             Order::get_instance()->update_status($order_number, Order::STATUS_PAID, [
                 'payment_method'         => 'card_pass',
                 'payment_transaction_id' => 'card_pass_' . $card_id,
@@ -502,7 +502,7 @@ class ChargeCard {
      * @param array<string,mixed> $meta
      */
     private function create_redeem_order(int $user_id, string $type, array $meta, int $card_id, string $password): string {
-        if (!class_exists('OrderRepository') || !class_exists('\AKSRT\Core\DomainModel')) {
+        if (!class_exists('AKSRT\\Payment\\OrderRepository') || !class_exists('\AKSRT\Core\DomainModel')) {
             return 'CARD' . $card_id . '_' . time();
         }
 
@@ -536,7 +536,7 @@ class ChargeCard {
             'product_id'     => $product_id,
             'amount'         => $amount,
             'currency'       => $currency,
-            'status'         => class_exists('Order') ? Order::STATUS_PENDING : 'pending',
+            'status'         => class_exists('AKSRT\\Payment\\Order') ? Order::STATUS_PENDING : 'pending',
             'payment_method' => 'card_pass',
             'note'           => is_string($note) ? $note : '',
             'expires_at'     => gmdate('Y-m-d H:i:s', time() + 30 * MINUTE_IN_SECONDS),