An order service with injected collaborators

Write OrderService(payments, orders, notifier). place_order(order_id, customer_id, lines) adds up the lines (quantity × unit_price, rounded to 2 places), charges the customer through payments.charge(customer_id, total) (which returns a payment reference or raises ValueError when the card is declined), saves the order with orders.save(order_id, total, reference), and tells the customer through notifier.send(customer_id, text). A declined payment saves nothing, sends "Order <id>: payment declined", and returns None. If saving fails, the error reaches the caller and no message is sent. Checks use fakes for all three collaborators.