Function measure_one_way_net_delay 

pub async fn measure_one_way_net_delay(
    backend: &BackendConn,
    user_id: u64,
    device_id: &str,
) -> Result<Duration>

Send one backend ping packet and return one-way delay.

Ref:

• FutuOpenD/Src/NNProtoCenter/Other/NNBiz_SvrTime.cpp:60-78
• FutuOpenD/Src/NNProtoCenter/Other/NNBiz_Ping.cpp:441-461

Rust does not currently have C++ INNData_AppInfo::GetNetType(), so net_type is intentionally omitted instead of defaulting to 0.