Attempt one commercial checkout and classify the result neutrally.
The request is validated before the adapter is called, so no provider can
be asked to charge an amount the application did not accept. A definitive
succeeded result is returned only when the adapter stated it; a thrown
adapter fault, a rejected shape, or an outcome outside the neutral contract
becomes an explicit unknown attempt that is neither paid nor failed.
Optionalcontext: OperationContext
Application use case that owns the provider-neutral checkout boundary.
The service depends only on the CheckoutGateway contract, so no provider protocol, authentication, or vocabulary can reach domain or application code. It is intentionally free of persistence: order finalization, sales contribution, and the durable attempt record belong to the later finalization slice, which calls this service.