GuestRegistrationInput

Identifies the guest being registered. The booker (the caller of registerForActivity) is recorded as the payer; the guest is the attendee.

Supply email and we reach the guest directly with the confirmation and every later notification; name is then optional and defaults to the email prefix (matching createUser behavior).

Omit email to book without contacting the guest — the surprise-gift case. The guest still gets their own attendee account and participant record (the booking is theirs, not the booker's), but the account carries no email address, so every notification for the booking falls back to the booker, exactly as it does for a managed child. name is required in this mode — it is all we have to identify the attendee by. An email can be attached to the account later, once the surprise has landed.