feat: Implement guest checkout functionality

[gustaavik]

• Updated CreateOrderInput to include session ID, email, phone number, and full name for guest checkouts.
• Enhanced CreateOrderFromCart method to handle both authenticated and guest checkouts.
• Added createOrderFromGuestCart method to process guest orders, including validation and cart retrieval.
• Modified Order and Cart entities to accommodate guest information and session IDs.
• Updated CartRepository to support retrieval by session ID and conversion of guest carts to user carts.
• Enhanced OrderRepository to handle guest orders, including new fields for guest information.
• Added unit tests for guest order creation and ensured cart and product stock updates are handled correctly.
• Created database migrations to support the new guest checkout structure.

[Copilot]

Pull Request Overview

This PR implements guest checkout functionality by updating order and cart repositories, use cases, and API endpoints to support guest users. Key changes include adding guest-specific fields to orders, enhancing cart handling to accommodate session IDs, and introducing new API routes and tests for guest orders and carts.

Reviewed Changes

Copilot reviewed 13 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
testutil/mock/cart_repository.go	Added guest cart support with session-based storage and thread safety.
internal/infrastructure/repository/postgres/order_repository.go	Updated SQL queries and result scanning to include guest order fields.
internal/infrastructure/repository/postgres/cart_repository.go	Modified cart repository to support guest cart conversion to user cart.
internal/domain/entity/order.go	Introduced NewGuestOrder and additional guest fields in Order.
internal/domain/entity/cart.go	Added NewGuestCart and updated JSON tags for guest carts.
internal/application/usecase/order_usecase.go	Created separate guest order flow handling and validations.
internal/application/usecase/cart_usecase.go	Added guest cart use case methods (get, add, update, remove, clear, convert).
internal/api/server.go	Added guest endpoints for cart and order operations.
internal/api/handler/order_handler.go	Updated order handler to support both authenticated and guest orders.
internal/api/handler/cart_handler.go	Integrated session cookie handling for guest cart operations.

Files not reviewed (2)

• migrations/000008_add_guest_checkout.down.sql: Language not supported
• migrations/000008_add_guest_checkout.up.sql: Language not supported