Function resolveConfirmOutcome

The §4a branching, as a pure function so both completion shapes stay testable without a browser:

• confirm-on-client (Stripe): the confirm result is already terminal.
• tokenize-first (Paysafe): confirm yields a clientToken; the host's onServerCompletion must call its own API route, which calls PaymentService.completePayment. Either way the caller sees one PayResult.