feat(offer-requests): expose split-ticket itineraries

[todor-a]

Exposes the split-ticket itineraries flow on POST /air/offer_requests.

• CreateOfferRequest.include_split_ticket?: boolean — opts into the per-slice fan-out.
• CreateOfferRequestQueryParameters.view?: 'offers' | 'itineraries' — itineraries returns offers grouped per slice / itinerary / fare brand; required to surface split-ticket candidates.
• New response types: OfferRequestItinerariesView, OfferRequestItinerariesViewSlice, Itinerary, ItineraryBrand, ItineraryOffer, ItineraryOfferType ('single_ticket' | 'split_ticket').
• OfferRequests.create returns a conditional type so view: 'itineraries' callers get the grouped shape; the existing return_offers: false discriminator is preserved.

Pricing and booking re-use the existing Offers.get / Orders.create resources.

const { data } = await duffel.offerRequests.create({
  cabin_class: 'economy',
  passengers: [{ type: 'adult' }],
  slices: [/* ... */],
  include_split_ticket: true,
  view: 'itineraries',
})
// data.slices[].itineraries[].brands[].offers[].type === 'split_ticket' | 'single_ticket'

Tests: 3 new specs in OfferRequests.spec.ts (body forwarding, query forwarding + response parsing, no view leak when omitted). Full suite 138/138, tsc --noEmit clean, lint clean.

Note: origin / destination on the itineraries-view slice are typed as Place to match OfferRequestSlice; the guide's abbreviated example shows bare ID strings — worth a check against a live response.