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/Sms/SMSProviderAliyun.php

@@ -82,7 +82,7 @@ class SMSProviderAliyun implements ProviderInterface {
      * @return array|\WP_Error
      */
     private function request(string $access_key, string $access_secret, string $region_id, array $biz_params) {
-        if (!class_exists('SMSSignatureHelper')) {
+        if (!class_exists('AKSRT\\Sms\\SMSSignatureHelper')) {
             return new \WP_Error('sms_helper_missing', '签名助手类未加载');
         }