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

tests/widget-system/user-features-contract.test.cjs

@@ -144,7 +144,6 @@ test('retired admin pages, schemas, APIs, and metadata chains are absent', () =>
     'assets/admin/src/constants/sections/extensions.js',
     'assets/admin/src/router/component-loaders/users.js',
     'assets/admin/src/router/component-loaders/tools.js',
-    'assets/admin/src/router/component-loaders/extensions.js',
     'assets/admin/src/api/modules/tools.js',
     'assets/admin/src/schema/sections/tools/index.js',
     'src/Admin/Schema/SchemaTools.php',