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

src/Admin/Schema/GlobalSchema.php

@@ -68,7 +68,7 @@ class GlobalSchema {
                     'groups'     => [
                         ['key' => 'basic', 'title' => 'SEO基础', 'fields' => [
                             ['key' => 'seo_enabled', 'label' => '启用核心SEO优化', 'type' => 'switch', 'default' => true, 'help' => '关闭后主题不再输出 SEO head 标签。'],
-                            ['key' => 'seo_ai_enabled', 'label' => 'AI生成SEO内容', 'type' => 'switch', 'default' => false, 'help' => '开启后文章/分类独立 SEO 框显示 AI 生成按钮；需配置 AI 连接（WP Connectors 或扩展 → AI 连接配置）。', 'dependsOn' => ['key' => 'seo_enabled', 'value' => true]],
+                            ['key' => 'seo_ai_enabled', 'label' => 'AI生成SEO内容', 'type' => 'switch', 'default' => false, 'help' => '开启后文章/分类独立 SEO 框显示 AI 生成按钮；需先在 WordPress 配置 AI Connectors。', 'dependsOn' => ['key' => 'seo_enabled', 'value' => true]],
                             ['key' => 'seo_connector', 'label' => 'SEO连接符', 'type' => 'input', 'default' => '-', 'help' => 'AI 生成标题时用于追加站点名。'],
                             ['key' => 'seo_title', 'label' => 'SEO标题(title)', 'type' => 'textarea', 'default' => ''],
                             ['key' => 'seo_keywords', 'label' => 'SEO关键字(keywords)', 'type' => 'textarea', 'default' => ''],