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/Ai/AIPrompt.php

@@ -77,31 +77,6 @@ class AIPrompt {
 6. 直接输出描述文本本身，不要任何前缀、引号或解释。';
     }
 
-    /**
-     * 兼容旧单 prompt 构建（草稿/无 ID 场景）。
-     */
-    public function build_seo_prompt(string $title, string $content): string {
-        $title_instruction = $this->get_seo_title_system_instruction();
-        $kw_instruction    = $this->get_seo_keywords_system_instruction();
-        $desc_instruction  = $this->get_seo_description_system_instruction();
-
-        return <<<PROMPT
-请根据以下文章内容，生成 SEO 优化建议。
-
-文章标题：{$title}
-
-文章内容：
-{$content}
-
-请以 JSON 格式返回，包含以下字段：
-- suggested_title: {$title_instruction}
-- suggested_description: {$desc_instruction}
-- suggested_keywords: {$kw_instruction}（以 JSON 数组形式返回）
-
-只返回 JSON，不要其他内容。
-PROMPT;
-    }
-
     public function build_summary_prompt(string $content, int $max_length = 200): string {
         $content = mb_substr($content, 0, 3000);
         return <<<PROMPT