PaymentProvider.resolveWebhookSession(event: WebhookEvent): Promise<ValidatedPaymentSession | "skip" | null>
Resolve a validated session from a webhook event. Each provider knows how to extract/fetch session data from its own event structure, so the webhook handler stays provider-agnostic.
event: WebhookEvent
Promise<ValidatedPaymentSession | "skip" | null>
the session, "skip" if the event should be acknowledged without processing (e.g. pending payment), or null on error.