test: add User fixture to smoke test null: false with RBS

Adds a User fixture demonstrating the null: false feature:
- id: required + non-null
- name: optional + non-null when present
- bio: optional + nullable

This fixture serves as both a test and documentation of the feature,
and ensures Steep type checking works correctly with the new option.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: hakanensari

test/fixtures/usage.rb

@@ -5,6 +5,7 @@
 require_relative "product"
 require_relative "tag_collection"
 require_relative "tree_node"
+require_relative "user"
 
 # Test that application code using Structure classes type-checks correctly. These should all pass Steep with no
 # warnings.
@@ -48,3 +49,13 @@
 tree_node.children
 tree_node.children&.first&.name
 tree_node.children&.first&.tags
+
+user = User.parse(id: "123", name: "Alice", bio: "Developer")
+user.id
+user.name
+user.bio
+
+user_without_bio = User.parse(id: "456", name: "Bob")
+user_without_bio.id
+user_without_bio.name
+user_without_bio.bio

test/fixtures/user.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+require "structure"
+
+User = Structure.new do
+  attribute(:id, String, null: false)
+  attribute?(:name, String, null: false)
+  attribute?(:bio, String)
+end

test/fixtures/user.rbs

@@ -0,0 +1,17 @@
+class User < Data
+  def self.new: (id: String?, ?name: String?, ?bio: String?) -> User
+              | (String?, String?, String?) -> User
+  def self.[]: (id: String?, ?name: String?, ?bio: String?) -> User
+             | (String?, String?, String?) -> User
+
+  def self.members: () -> [ :id, :name, :bio ]
+
+  def self.parse: (?Hash[String | Symbol, untyped], **untyped) -> User
+
+  attr_reader bio: String?
+  attr_reader id: String?
+  attr_reader name: String?
+
+  def members: () -> [ :id, :name, :bio ]
+  def to_h: () -> { id: String?, name: String?, bio: String? }
+end

test/test_rbs.rb

@@ -10,6 +10,7 @@
 require_relative "fixtures/product"
 require_relative "fixtures/tag_collection"
 require_relative "fixtures/tree_node"
+require_relative "fixtures/user"
 
 class TestRBS < Minitest::Test
   def setup
@@ -64,6 +65,13 @@ def test_emit_rbs_mixed_array_and_self_referential
     assert_equal(expected.strip, actual.strip)
   end
 
+  def test_emit_rbs_non_nullable
+    expected = File.read("test/fixtures/user.rbs")
+    actual = Structure::RBS.emit(User)
+
+    assert_equal(expected.strip, actual.strip)
+  end
+
   def test_emit_rbs_non_data_class
     result = Structure::RBS.emit(String)