Function fetch_main_brokers 

pub async fn fetch_main_brokers(
    backend: &BackendConn,
) -> Result<MainBrokerSnapshot>

发 CMD9419, 解析 response 成 MainBrokerSnapshot.

失败模式:

• 网络错误 / decode 错 → Err (caller log + 继续, 不阻塞 daemon 启动)
• ret_code != 0 → Err with backend err_message

行为对齐 C++ NNProto_Trd_MainBrokerage::PullMainBrokerage() 全 path.