Builder

Builder constructs a complex object step by step via a fluent interface,
instead of one constructor call with a dozen parameters.

Implement PizzaBuilder with set_size(size), add_topping(topping)
(both return self for chaining) and build() returning the finished
Pizza. Pizza.describe() returns a human-readable summary.

solve(orders) builds one pizza per order dict {'size':..., 'toppings': [...]}
and returns each describe() string.