Add a Berlin City Tour ticket to the basket for a date

Automatically add a Berlin City Tour ticket to the basket for a date on berlin-city-tour.de. Puts tickets for one travel date into the Berlin City Tour basket and returns the basket contents it created. Takes the guest counts per price category, as check_availability reports them, and the offer_hash from that same check. Before it writes anything it re-reads what the shop sells on that date and recomputes the hash: if the date, the timeslots or the prices have moved since the check, it adds nothing and refuses with what the shop shows now, so the caller can confirm the new offer and retry. It also refuses a date the shop does not sell, a guest category the product does not offer, and a request for more guests than the remaining capacity. The basket lives in the browser, so the cartPackageGroupId it returns is what the later booking steps work from.