该接口用于查询指定证券账号对特定标的的可卖空数量。通过传入账号 account_no 及股票代码 symbol,返回当前可用于卖空操作的 最大股票数量。This interface is used to query the available short-sellable quantity for a specific symbol in a specified securities account. By providing the account number account_no and stock symbol symbol, it returns the maximum number of shares currently available for short selling.