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/Ai/AISEO.php

@@ -42,7 +42,7 @@ class AISEO {
             ];
         }
 
-        if (!class_exists('AI') || !class_exists('AIPrompt')) {
+        if (!class_exists('AKSRT\\Ai\\AI') || !class_exists('AKSRT\\Ai\\AIPrompt')) {
             return [
                 'success' => false,
                 'message' => __('AI 模块未加载', 'aksrt'),
@@ -115,7 +115,7 @@ class AISEO {
             return ['success' => false, 'message' => __('AI SEO 功能未启用', 'aksrt')];
         }
 
-        if (!class_exists('AI') || !class_exists('AIPrompt')) {
+        if (!class_exists('AKSRT\\Ai\\AI') || !class_exists('AKSRT\\Ai\\AIPrompt')) {
             return ['success' => false, 'message' => __('AI 模块未加载', 'aksrt')];
         }