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');
         }
 

src/AKSRT/Sms/SMSProviderFactory.php

@@ -52,13 +52,13 @@ class SMSProviderFactory {
     private static function create(string $key): ?ProviderInterface {
         switch ($key) {
             case 'aliyun':
-                return class_exists('SMSProviderAliyun') ? new SMSProviderAliyun() : null;
+                return class_exists('AKSRT\\Sms\\SMSProviderAliyun') ? new SMSProviderAliyun() : null;
             case 'tencent':
-                return class_exists('SMSProviderTencent') ? new SMSProviderTencent() : null;
+                return class_exists('AKSRT\\Sms\\SMSProviderTencent') ? new SMSProviderTencent() : null;
             case 'huawei':
-                return class_exists('SMSProviderHuawei') ? new SMSProviderHuawei() : null;
+                return class_exists('AKSRT\\Sms\\SMSProviderHuawei') ? new SMSProviderHuawei() : null;
             case 'custom':
-                return class_exists('SMSProviderCustom') ? new SMSProviderCustom() : null;
+                return class_exists('AKSRT\\Sms\\SMSProviderCustom') ? new SMSProviderCustom() : null;
             default:
                 return null;
         }