Function build_fin_real 

pub fn build_fin_real(
    req_id: String,
    acc_id: u64,
    cipher: Vec<u8>,
    security_type: Option<u32>,
    sub_account_id: Option<u64>,
) -> MsgHeader

Construct the FIN service real-account header used by Desktop BFF and position-action commands. These commands own a durable request id, so the caller supplies it instead of generating a transport-local id here.