Add null: false option for GraphQL-style non-null semantics

[hakanensari]

Summary

Implements #15 by adding a null keyword parameter to both attribute and attribute? methods. When set to false, Structure will raise an ArgumentError if the coerced value is nil.

Usage

User = Structure.new do
  attribute(:id, String, null: false)              # required key, must be non-null
  attribute?(:name, String, null: false)           # optional key, but must be non-null when present
  attribute?(:description, String)                 # optional key, can be null (default)
end

User.parse(id: "123", name: "Alice")               # ✓ valid
User.parse(id: "123", name: nil)                   # ✗ raises ArgumentError: cannot be null: :name
User.parse(id: "123")                              # ✓ valid (name is optional)

Changes

• Add @non_nullable Set to Builder to track null: false attributes
• Add null: true parameter to attribute and attribute? methods
• Add Builder#non_nullable accessor method
• Update Structure metadata to include non_nullable
• Add validation in parse method after coercion
• Update RBS type signatures
• Add comprehensive test suite (18 tests)
• Add User fixture for RBS smoke testing

Design Decisions

• Default null: true: Backward compatible, most attributes allow nil
• Post-coercion validation: Catches nil from payload, defaults, or coercion results
• Works with optional attributes: attribute?(:name, String, null: false) raises only if key exists but value is nil
• GraphQL semantics: Matches graphql-ruby vocabulary perfectly
• Error message: "cannot be null: :attr" - lowercase, consistent with parse-time errors

Test Coverage

• Required + null: false (missing key vs nil value)
• Optional + null: false (missing allowed, nil not allowed)
• Default values that are nil
• Coercion results that are nil
• Block transformations that return nil
• Mixed nullable and non-nullable attributes
• Nested structures and arrays

Closes #15

🤖 Generated with Claude Code