Fix Sum type to_s with Dry::Struct types

[baweaver]

Fix Sum type to_s with Dry::Struct types

Fixes #482

Problem

Sum types composed of two Dry::Struct types raise NoMethodError: undefined method 'visit_sum_constructors' when calling to_s.

Solution

Added missing visit_sum_constructors method to the Printer class that recursively handles sum type printing by visiting left and right branches.

Changes

• Add visit_sum_constructors method that handles sum type decomposition
• Add visit_sum_constructor helper method for recursive sum type handling
• Add test coverage for sum types with Dry::Struct types

Testing

# This now works without error:
class A < Dry::Struct; end
class B < Dry::Struct; end

(A | B).to_s