Validate

Validates and processes loyalty points redemption within a shopping cart context. It retrieves the user associated with the request, then iterates through the items in the cart, verifies their existence in the merchant's catalogue, and builds a redemption cart accordingly. Afterward, it calculates the points offer for redemption and handles potential exceptions such as exceeding the points limit or encountering redemption errors. Finally, it returns a success response containing details such as the user's available points, the value of points to be redeemed, and the maximum points available for redemption.