Initializes a shopping cart with a selected eSIM package.

This is the first step in the checkout flow for new eSIM purchases. It reserves the package and generates a cart_key which you will use in the final submission step.

Date rules:

• Both start_date and end_date are required.
• start_date must be today or in the future.
• end_date must be after start_date.
• The number of days between start_date and end_date must not exceed the package validity_days returned by GET /esim/destination/{destination_code}/packages.