processBooking(listing: ListingWithCount,contact: ContactInfo,quantity: number,date: string | null,baseUrl: string,customUnitPrice?: number,answerIds?: number[],): Promise<BookingResult>
Process a single-listing booking.
Resolves the payment plan, then either:
- Creates a checkout session (paid) or
- Atomically creates an attendee (free or payment provider disabled)
listing: ListingWithCount
contact: ContactInfo
Promise<BookingResult>