Backend Feature: Checkout

User story

As a visitor, I want to checkout my cart so that I can send my order to the institute via WhatsApp.

Dependencies

• Entities: Cart, CartItem, Product, Institution
• API specs: POST /cart/checkout

Acceptance criteria

• [ ] Finds the open cart for the given sessionId; returns 404 if none exists.
• [ ] Returns 422 if the cart has no items.
• [ ] Fetches institution WhatsApp; returns 422 if not set.
• [ ] Sets cart status to closed.
• [ ] Builds and returns a whatsappUrl with a pre-filled message listing products, quantities, unit prices, and total.

Edge cases

• No open cart for sessionId: returns 404.
• Open cart exists but has no items: returns 422.
• Institution has no WhatsApp: returns 422.

Notes

• Cart is closed even if the WhatsApp URL cannot be built? No — close the cart only after confirming the institution has a WhatsApp, to avoid losing the cart without a usable URL.
• Total is calculated as the sum of quantity * price for each item.
• Prices are formatted with two decimal places in the message (e.g. R$ 29,90).