该接口用于查询指定证券账号的当前购买力。通过传入账号 account_no 及可选币种 currency,返回该账号在指定币种下的最大可用购买力数值。购买力综合考虑账户现金余额与融资额度。This interface is used to query the current buying power of a specified securities account. By providing the account number account_no and an optional currency currency, it returns the maximum available buying power for the account in the specified currency. The buying power takes into account both the account's cash balance and financing limit.