Function build_real_trade_account_heartbeat 

pub fn build_real_trade_account_heartbeat() -> MsgHeader

Build the global account-list heartbeat header.

C++ deliberately writes only req_id, seeded with 1 for real trading; account id and cipher remain absent because the repeated account list owns the subscription identity. Ref: NNProto_Trd_KeepAlive.cpp:84-95.