Document required bookable.price and the 400 on POST /{version}/bookings

[lrcarrera]

Description

Companion to getyourguide/public-partner-api#2105.

POST /{version}/bookings without data.booking.bookable.price used to fail with a 500 (NullPointerException in the price check). It now returns a 400 (errorCode 12, Booking price is required.) before any availability or pricing lookup.

Changes:

• BookingsCreate description (spec/paths/bookings.yaml): spell out the required bookable fields (option_id, datetime, price, categories), the 400 for a missing price, and that a present price is then checked against the option's current price (errorCode 308).

No schema or type changes: price was already listed under required for bookable, so generated clients are unaffected. Description-only, same shape as #125.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works (n/a, description-only spec change; behavior is covered by unit + Karate tests in the linked service PR)

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