fix: Prevent ActiveRecord constant leak in uniqueness matcher (thoughtbot#1694)

When using the validate_uniqueness_of matcher, test model classes were
being added to ActiveRecord::Base.descendants and not being removed
after the matcher ran. This could lead to unexpected behavior in tests
that rely on the list of ActiveRecord descendants.

Closes thoughtbot#1663

Author: matsales28

lib/shoulda/matchers/active_record/uniqueness/namespace.rb

@@ -10,16 +10,33 @@ def initialize(constant)
           end
 
           def has?(name)
-            constant.const_defined?(name)
+            constant.const_defined?(name, false)
           end
 
           def set(name, value)
             constant.const_set(name, value)
           end
 
           def clear
+            test_model_classes = []
+
             constant.constants.each do |child_constant|
-              constant.__send__(:remove_const, child_constant)
+              child = constant.const_get(child_constant)
+
+              if defined?(::ActiveRecord::Base) &&
+                 child.is_a?(Class) &&
+                 child < ::ActiveRecord::Base
+                test_model_classes << child
+              end
+
+              constant.remove_const(child_constant)
+            rescue NameError
+              # Constant may have been removed elsewhere; ignore
+            end
+
+            if defined?(::ActiveSupport::DescendantsTracker) &&
+               test_model_classes.any?
+              ::ActiveSupport::DescendantsTracker.clear(test_model_classes)
             end
           end
 

spec/unit/shoulda/matchers/active_record/validate_uniqueness_of_matcher_spec.rb

@@ -1539,6 +1539,35 @@ def name=(name)
     end
   end
 
+  context 'test model cleanup' do
+    it 'removes test models from ActiveRecord::Base.descendants after validation' do
+      descendants_before = ActiveRecord::Base.descendants.map(&:to_s)
+
+      model = define_model :example, owner_id: :integer, owner_type: :string do |m|
+        m.belongs_to :owner, polymorphic: true
+        m.validates_uniqueness_of :owner_id, scope: :owner_type
+      end
+
+      existing_owner_model = define_model(:user)
+      owner = existing_owner_model.create!
+      model.create!(owner:)
+
+      record = model.new
+
+      expect(record).to validate_uniqueness_of(:owner_id).scoped_to(:owner_type)
+
+      descendants_after = ActiveRecord::Base.descendants.map(&:to_s)
+      new_descendants = descendants_after - descendants_before
+
+      test_model_descendants = new_descendants.select do |name|
+        name.start_with?('Shoulda::Matchers::ActiveRecord::Uniqueness::TestModels::')
+      end
+
+      expect(test_model_descendants).to be_empty,
+        "Expected no test models in ActiveRecord::Base.descendants, but found: #{test_model_descendants.join(', ')}"
+    end
+  end
+
   let(:model_attributes) { {} }
 
   def default_attribute