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

templates/pages/page-submit.php

@@ -106,11 +106,6 @@ if (file_exists($js)) {
 		],
 
 		'imageUploadMultiple' => $image_upload_multiple,
-		'ai'        => [
-			'available' => !empty($ctx['ai_available']),
-			'summary'   => !empty($ctx['ai_available']),
-			'tags'      => !empty($ctx['ai_available']),
-		],
 		'editor'    => [
 			'canUploadImage' => !empty($ctx['can_upload_img']),
 			'canUploadVideo' => !empty($ctx['can_upload_video']),
@@ -142,10 +137,6 @@ if (file_exists($js)) {
 			'submitted'     => __('投稿已提交', 'aksrt'),
 			'error'         => __('操作失败，请重试', 'aksrt'),
 			'confirmDelete' => __('确定要删除这个草稿吗？', 'aksrt'),
-			'aiGenerating'  => __('生成中…', 'aksrt'),
-			'aiFailed'      => __('AI 生成失败', 'aksrt'),
-			'needContent'   => __('请先填写正文内容', 'aksrt'),
-			'aiFilled'      => __('已填入，可继续编辑', 'aksrt'),
 			'deleting'      => __('正在删除草稿…', 'aksrt'),
 			'deleted'       => __('草稿已删除', 'aksrt'),
 			'uploading'     => __('正在上传…', 'aksrt'),