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/toast-contract.test.cjs

@@ -379,8 +379,6 @@ test('front submit actions use direct fixed-id notyf flows', () => {
     'aksrt-front-submit-delete-draft-toast',
     'aksrt-front-submit-resource-upload-toast',
     'aksrt-front-submit-editor-media-toast',
-    'aksrt-front-submit-ai-tags-toast',
-    'aksrt-front-submit-ai-summary-toast',
   ].forEach((id) => assert.match(source, new RegExp("['\"]" + id + "['\"]")));
 
   assert.match(submit, /submit: function[\s\S]*?notyf\([^;]+?'load'[^;]+?toastId[\s\S]*?notyf\([^;]+?'success'[^;]+?toastId[\s\S]*?notyf\([^;]+?'danger'[^;]+?toastId/);
@@ -398,8 +396,6 @@ test('front submit actions use direct fixed-id notyf flows', () => {
   // 付费下载附件选择后仅补一条成功确认，上传过程通知属于选择器。
   assert.match(downloads, /AksrtSubmitMedia\.open\([\s\S]*?'success', 5, 'aksrt-front-submit-resource-upload-toast'/);
   assert.match(editor, /aksrt-front-submit-editor-media-toast[\s\S]*?'load'[\s\S]*?'success'[\s\S]*?'danger'/);
-  assert.match(submit, /aksrt-front-submit-ai-tags-toast[\s\S]*?aksrt-front-submit-ai-summary-toast[\s\S]*?'load'[\s\S]*?'success'[\s\S]*?'danger'/);
-  assert.match(submit, /needContent[^;]+?'warning'/);
   assert.doesNotMatch(submit, /\b(?:window\.)?alert\s*\(/);
 
   assert.match(page, /'aksrt-front-submit',[\s\S]*?'\/assets\/js\/front-submit\.js',[\s\S]*?\[[^\]]*'loader_js'[^\]]*\]/);