Fix JSON schema generation for Dry::Struct wrapped in constructors

[baweaver]

Fix JSON schema generation for Dry::Struct wrapped in constructors

Fixes #495

Problem

When generating JSON schema for a schema containing a Dry::Struct wrapped in a constructor (e.g.,
Address.constructor(&:itself)), all struct properties were omitted from the generated schema, returning only
{type: "object"} instead of the full schema with properties.

Root Cause

The struct? method in the struct extension only recognized direct struct classes (Class instances that inherit
from Dry::Struct), but not struct constructors (Dry::Types::Constructor instances wrapping structs).

Solution

• Extended struct? method to also recognize Dry::Types::Constructor instances that wrap struct classes
• Added extract_struct_class method to extract the underlying struct class from both direct structs and
constructors
• Modified the macro processing to use the extracted struct class for schema compilation

Changes

• lib/dry/schema/extensions/struct.rb: Enhanced struct detection and extraction logic
• spec/integration/extensions/json_schema/struct_constructor_spec.rb: Comprehensive test coverage
• CHANGELOG.md: Document the fix

Before/After

# Before: Missing struct properties
Dry::Schema.Params do 
  required(:address).value(Address.constructor(&:itself)) 
end.json_schema
# => {:properties=>{:address=>{:type=>"object"}}} # No properties

# After: Full struct schema included  
Dry::Schema.Params do 
  required(:address).value(Address.constructor(&:itself)) 
end.json_schema
# => {:properties=>{:address=>{:type=>"object", :properties=>{:street=>{...}}}}} # Properties included

Testing

• Tests direct struct usage (still works)
• Tests struct constructor usage (now works)
• Verifies both generate schemas with proper struct properties