Function backend_desired_key_for_sec_key 

pub fn backend_desired_key_for_sec_key(
    sec_key: &str,
    stock_id: u64,
    sec_type: i32,
    mkt_id: u32,
) -> Option<(u64, i32)>

Derive the backend desired-set key for a cached subscription row.

Qot_Sub stores subscriptions under the public FTAPI sec_key, but backend CMD6211 desired set must use the precise backend quote market for futures. This helper keeps the “parse public key, then derive backend market from cache sec_type/mkt_id” rule in one place for normal unsubscribe and unsub-all paths.