Synchronize (replace) the line items in a cart.

This endpoint creates a new cart or updates an existing one by replacing all current
line items with the provided items.

Cart resolution logic:

• If cart_id is provided in the body, the cart with that ID is updated directly.
• If customer_id is provided (without cart_id), the system looks for an existing
  active cart for that customer. If found, it replaces the lines; otherwise, a new cart is created.
• If neither cart_id nor customer_id is provided, a new cart is created.

Required fields:

• interaction_id (the current interaction/conversation ID)
• cart_lines (at least one item)

A successful request returns the cart with the updated line items.