Function single_crypto_account_broker 

pub fn single_crypto_account_broker(trd_cache: &TrdCache) -> Option<u32>

v1.4.110 codex audit P1 #3: 用户唯一已开户 crypto account 的 broker_id.

对齐 C++ INNData_Trd_MainBrokerage::GetCryptoSupportedDefaultMainBroker (line 70-123) 优先级 1: 如果只开了一个 crypto account, 直接取该 account 的 broker.

返:

• Some(broker_id): trd_cache 恰好 1 个 is_crypto_account(), 取其 broker
• None: 0 个或 ≥ 2 个 crypto account, caller 走 9419 crypto_brokers / fallback 路径

此值作 resolve_qot_broker_for_request / resolve_or_reject_broker 第 5 参数注入, QOT handler securityFirm=Unknown(0) 时决定 default broker.