Create checkout session

Create an 'open' checkout session to start a purchase.

Contract modes:

• Plan-based: set contract.is_plan_based = true and provide plan_id, currency, start_date.
• Product-based: set contract.is_plan_based = false and provide currency, start_date, and phases with pricings.

Customer modes:

• Customer-based: provide customer_id of an existing customer.
• Guest: set guest_checkout = true and provide customer details and business_entity_id.

Validation notes:

• currency must be ISO-4217 and match the contract currency if provided.
• success_url, cancel_url, pending_url must be fully qualified URLs.
• idempotency_key allows safe retries without duplicates.

Response: returns the created session (status 'open') and pricing estimates following EstimationResponseSchema. See the examples for plan-based and product-based requests and responses.