refactor(admin): drop AI connection config page for WP AI Connectors

Remove the "AI 连接配置" page under 扩展&增强 together with the theme's
own OpenAI-compatible provider path. AI generation (SEO, summary, tags)
now runs solely on the WordPress AI Client (wp_ai_client_prompt), with
connections configured through WordPress AI Connectors.

- AI.php keeps only the WP AI Client path: no provider config storage,
  no theme fallback, no rate limiting
- Drop /ai/status, /ai/settings and legacy /ai/seo-suggest routes plus
  their handlers
- Remove dead compat code: AISEO::suggest, AIPrompt::build_seo_prompt
  and the front-submit AI assist flow (its buttons were never rendered
  and its endpoints were never registered)
- Update SEO schema/meta-box guidance that pointed at the deleted page
- Add ai-config-removal contract test and refresh the toast and
  user-features contracts affected by the cleanup

src/Core/SEOMetaBox.php

@@ -210,16 +210,16 @@ class SEOMetaBox {
             true
         );
 
-        $status = class_exists('\AKSRT\Ai\AI') ? \AKSRT\Ai\AI::get_instance()->get_status() : [];
-        $seo    = class_exists('\AKSRT\Settings\Settings') ? \AKSRT\Settings\Settings::get_seo_settings() : [];
+        $ai_available = class_exists('\AKSRT\Ai\AI') && \AKSRT\Ai\AI::get_instance()->is_available();
+        $seo          = class_exists('\AKSRT\Settings\Settings') ? \AKSRT\Settings\Settings::get_seo_settings() : [];
 
         wp_localize_script('aksrt-admin-ai-seo', 'aksrtAiSeo', [
             'restUrl' => esc_url_raw(rest_url('aksrt/v1')),
             'nonce'   => wp_create_nonce('wp_rest'),
             'ai'      => [
-                'available'      => !empty($status['available']),
+                'available'      => $ai_available,
                 'seo_ai_enabled' => !empty($seo['seo_ai_enabled']) && !empty($seo['seo_enabled']),
-                'write_assist'   => !empty($status['available']),
+                'write_assist'   => $ai_available,
             ],
             'i18n'    => [
                 'generating'        => __('生成中…', 'aksrt'),
@@ -262,7 +262,7 @@ class SEOMetaBox {
         $available = class_exists('\AKSRT\Ai\AI') && \AKSRT\Ai\AI::get_instance()->is_available();
         $disabled  = (!$available || $id <= 0) ? ' disabled' : '';
         $title     = !$available
-            ? esc_attr__('请先配置 AI 连接（扩展 → AI 连接配置 或 WP Connectors）', 'aksrt')
+            ? esc_attr__('请先配置 WordPress AI Connectors', 'aksrt')
             : ($id <= 0 ? esc_attr__('请先保存后再生成', 'aksrt') : '');
 
         return sprintf(