Fix inline Packer assignment to use Shakapacker directly

- Changes Packer = ReactOnRails::PackerUtils.packer to Packer = Shakapacker
- Simplifies test setup by directly using Shakapacker constant
- Removes unnecessary indirection through PackerUtils.packer method
- Aligns with modern codebase standards

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

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

Author: justin808

spec/react_on_rails/test_helper/webpack_assets_status_checker_spec.rb

@@ -61,7 +61,7 @@
         let(:fixture_dirname) { "assets_with_manifest_exist_server_bundle_separate" }
 
         before do
-          Packer = ReactOnRails::PackerUtils.packer # rubocop:disable Lint/ConstantDefinitionInBlock, RSpec/LeakyConstantDeclaration
+          Packer = Shakapacker # rubocop:disable Lint/ConstantDefinitionInBlock, RSpec/LeakyConstantDeclaration
           allow(ReactOnRails::PackerUtils).to receive_messages(
             manifest_exists?: true,
             packer_public_output_path: generated_assets_full_path