Function build_crypto 

pub fn build_crypto(acc_id: u64, cipher: Vec<u8>) -> CryptoMsgHeader

构造 crypto trade_cmn::CryptoMsgHeader (仅 3 字段).

空 cipher 处理为 None 而非 Some(vec![]) (对齐 v1.4.110 之前 crypto_trade.rs::build_crypto_msg_header 原行为, C++ 参照见 NNProto_Trd_OrderOpCrypto.cpp:40-48).