Return the checkout session bound to the guest session token or, after account creation, to the authenticated traveler account. This polling surface reports booking progress through the booked state.

Second-device guarantee: once POST /v1/flights/bookings returns 201 for this account's account_created session, every later call here — even from a device that never saw the create response — returns either bookingId set to that booking's id or status: booked. Use this to detect an already-created booking and resume at confirm instead of creating and charging a second one; see CheckoutSessionOut.bookingId for the full guarantee.