Fix mock objects in packer_utils_spec

Use double() instead of instance_double() with class constants
that may not be loaded in test environment.

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

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

Author: justin808

spec/react_on_rails/packer_utils_spec.rb

@@ -92,8 +92,8 @@ module ReactOnRails
     end
 
     describe ".supports_auto_registration?" do
-      let(:mock_config) { instance_double(Shakapacker::Config) }
-      let(:mock_packer) { instance_double(Shakapacker, config: mock_config) }
+      let(:mock_config) { double("MockConfig") }
+      let(:mock_packer) { double("MockPacker", config: mock_config) }
 
       before do
         allow(described_class).to receive(:packer).and_return(mock_packer)