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/router/component-loaders/extensions.js

@@ -1,3 +0,0 @@
-export const extensionsPageLoaders = {
-  ExtensionAI: () => import('@/pages/extensions/ExtensionAI.vue'),
-}

assets/admin/src/router/component-loaders/index.js

@@ -1,11 +1,9 @@
 import { corePageLoaders } from './core'
 import { usersPageLoaders } from './users'
 import { toolsPageLoaders } from './tools'
-import { extensionsPageLoaders } from './extensions'
 
 export const componentLoaders = {
   ...corePageLoaders,
   ...usersPageLoaders,
   ...toolsPageLoaders,
-  ...extensionsPageLoaders,
 }