Create a new checkout session to initiate a billing flow.

Checkout sessions are temporary objects that allow customers to subscribe to plans, change plans, or update quantities. They generate a preview of the contract and pricing that will be applied when the checkout is confirmed.

Checkout Type Detection: The checkout type is automatically determined based on the provided fields:

• New Subscription (plan_based): Provide plan_id without contract_id
• Plan Change (coming soon): Provide both contract_id and plan_id
• Quantity Update (coming soon): Provide contract_id with product_quantities but no plan_id

Billing Estimates: The response includes simplified billing estimates with:

• current_invoice: Charges for the current billing period
• next_invoice: Upcoming charges for the next period (null if none) Estimates are generated as a preview and do not create actual invoices.

The checkout session expires after 24 hours if not completed.