Confirm a checkout session and create the actual contract.

This endpoint transitions a checkout from preview/pending state to confirmed, creating the actual contract in the system.

Prerequisites:

• Checkout must be in 'pending' or 'payment_succeeded' status
• Checkout must not be expired

Payment Method:

• Optional payment_method_id can be provided
• If provided, it will be stored with the checkout

Flow:

1. Validate checkout state
2. Create contract from checkout preview
3. Update checkout status to 'contract_created'
4. Return confirmed checkout details