feat: 智能Agent接入前连通性/鉴权校验

- 前端技能市场「接入单个Agent」新增「测试连接」按钮，接入前必须校验通过

- 后端 POST /market/apps/verify-agent：dify 走 chat-messages 鉴权探测(trust_env=False 直连)，n8n/coze 走 HEAD 可达性探测

- upload_app 对 agent 类型做服务端守卫，密钥无效/不可达直接 400 拒绝

- MarketApp 新增 platform 列(dify/n8n/coze)，009 迁移幂等加列

frontend/src/api/index.ts

@@ -313,6 +313,8 @@ export const marketAPI = {
 
   createApp: (data: any) => post<any>('/market/apps', data),
 
+  verifyAgent: (data: any) => post<any>('/market/apps/verify-agent', data),
+
   purchaseApp: (id: string) => post<any>(`/market/apps/${id}/purchase`),
 
   installApp: (id: string) => post<any>(`/market/apps/${id}/install`),