Request body for creating a trip.

Two usage modes:

• Direct — supply purpose (and optionally startsAt/endsAt)
  with no bookingId. The supplied value is stored as the traveler's
  custom label (purpose, so later booking facts never retitle it) and
  as defaultPurpose; clearing purpose via PATCH reverts to the
  automatic title.
• Booking-linked — supply bookingId to atomically create a trip and
  associate an existing standalone booking. defaultPurpose is set to the
  booking's destination city. purpose, when non-blank, is stored as the
  traveler's initial custom label and immediately reflected in responses.
  bookingId is the Mercury Booking.id UUID.

A future ticket will introduce a stages field aligned with the
MER-449 trip-stage shape. The pre-MER-449 segments field has been
removed — use POST /v1/trips/{tripId}/bookings to attach bookings
after creation.