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/api/modules/extensions.js

@@ -5,11 +5,4 @@ export const extensionsApiMethods = {
 
   testIpGeo(ip) { return this.request('/extensions/ip-test', { method: 'POST', data: { ip } }) },
   testContentAudit(text) { return this.request('/extensions/audit-test', { method: 'POST', data: { text } }) },
-
-  /* AI 辅助 */
-  getAiStatus() { return this.request('/ai/status') },
-  updateAiSettings(data) { return this.request('/ai/settings', { method: 'POST', data }) },
-  generateSeo(data) { return this.request('/ai/seo-generate', { method: 'POST', data }) },
-  suggestSummary(data) { return this.request('/ai/summary-suggest', { method: 'POST', data }) },
-  suggestTags(data) { return this.request('/ai/tags-suggest', { method: 'POST', data }) },
 }