Function fetch_acc_cash_flow_single 

async fn fetch_acc_cash_flow_single(
    gateway: &str,
    acc_id: u64,
    clearing_date: &str,
    env: &str,
    market: &str,
    direction: Option<i32>,
) -> Result<Vec<FlowSummaryInfo>>

拆出的单日拉取 helper：单日版和 range 版共用。