Create a Stripe Checkout URL after customer approval

The agent must show the customer the final quote and obtain explicit approval first. This operation only creates a Stripe Checkout URL. It does not charge the customer. The customer supplies payment details directly to Stripe, which places a temporary authorization hold. AIURION captures the payment only after manually approving the submitted model. The Idempotency-Key is durably bound to the quote and approved checkout selections; retrying the same payload replays the original result, while reusing the key with a different payload returns a conflict.