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

assets/admin/src/constants/sections/extensions.js

@@ -1,4 +1,4 @@
-import { renderFaIcon, schemaPage, customPage } from './helpers'
+import { renderFaIcon, schemaPage } from './helpers'
 
 export const extensionsSection = {
   key: 'extensions',
@@ -56,15 +56,5 @@ export const extensionsSection = {
       canonicalDomain: 'extensions',
       section: 'security',
     }),
-    customPage({
-      key: 'extensions-ai',
-      name: 'ExtensionAI',
-      label: 'AI 连接配置',
-      route: '/extensions/ai',
-      component: 'ExtensionAI',
-      status: 'available',
-      title: 'AI 连接配置',
-      desc: '配置 AI 生成连接；SEO 在文章独立 SEO 框，摘要/标签在前台投稿页。',
-    }),
   ],
 }