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/schema/sections/global/seo.js

@@ -2,7 +2,7 @@ export const seoSchema = {
   key: 'global.seo',
   title: 'SEO 优化',
   icon: 'fa fas fa-search',
-  description: '站点标题、描述、关键词与 AI SEO。AI 生成入口在文章「独立 SEO」框；连接配置在「扩展 → AI 连接配置」。优先使用 WordPress AI Connectors。',
+  description: '站点标题、描述、关键词与 AI SEO。AI 生成入口在文章「独立 SEO」框，连接由 WordPress AI Connectors 提供。',
   defaults: {
     seo_enabled: true,
     seo_ai_enabled: false,
@@ -48,7 +48,7 @@ export const seoSchema = {
           label: '启用 AI SEO 生成',
           type: 'switch',
           default: false,
-          help: '开启后文章/分类「独立 SEO」框显示「AI 生成 SEO」按钮。需先配置 AI 连接（WP Connectors 或扩展 → AI 连接配置）。',
+          help: '开启后文章/分类「独立 SEO」框显示「AI 生成 SEO」按钮。需先在 WordPress 配置 AI Connectors。',
           dependsOn: { key: 'seo_enabled', value: true },
         },
         { key: 'seo_connector', label: '标题连接符', type: 'input', default: '-', placeholder: '例如：- 或 |', help: 'AI 生成标题时会用此连接符追加站点名。' },