POST /api/v1/checkout/ always converted the customer's entire
multi-store cart into orders. Add an optional store_uuids field to
CheckoutSerializer — when given, checkout() only processes those
stores' cart groups and only clears their items, leaving the rest of
the cart intact for a later checkout. Omitted, behavior is unchanged
(whole cart checks out).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
