Fix PackageJson constant NameError in generator_helper_spec

The test was failing with:
  NameError: uninitialized constant PackageJson

This occurred because instance_double(PackageJson) requires the
PackageJson constant to be defined at the time the double is created.

Solution: Stub the PackageJson constant in the before block before
creating the instance_double, consistent with other tests in the same
file that use stub_const for PackageJson.

This fix resolves all 3 failing tests in CI:
- calls manager.add with exact: true
- calls manager.add with type: :dev and exact: true
- returns false and prints a warning

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

Co-Authored-By: Claude <[email protected]>

Author: justin808

spec/react_on_rails/generators/generator_helper_spec.rb

@@ -13,6 +13,9 @@
       let(:mock_manager) { instance_double("PackageJson::Manager") } # rubocop:disable RSpec/VerifiedDoubleReference
 
       before do
+        # Stub PackageJson constant so instance_double can reference it
+        stub_const("PackageJson", Class.new) unless defined?(PackageJson)
+
         allow(self).to receive(:package_json).and_return(mock_package_json)
         allow(mock_package_json).to receive(:manager).and_return(mock_manager)
       end