Fix CI failures by restoring packer availability check

- Restores using_packer? check to handle test scenarios where packer is disabled
- Combines manifest.json check with packer availability in cleaner conditional
- Reverts one inline Packer assignment that breaks when Shakapacker isn't loaded
- Maintains original functionality while keeping code improvements

All failing tests now pass:
- webpack_assets_status_checker_spec.rb (all scenarios)
- utils_spec.rb "without a packer enabled" scenarios

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

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

Author: justin808

lib/react_on_rails/utils.rb

@@ -72,7 +72,7 @@ def self.bundle_js_file_path(bundle_name)
       # Priority order depends on bundle type:
       # SERVER BUNDLES (normal case): Try secure non-public locations first, then manifest, then legacy
       # CLIENT BUNDLES (normal case): Try manifest first, then fallback locations
-      if bundle_name == "manifest.json"
+      if bundle_name == "manifest.json" || !ReactOnRails::PackerUtils.using_packer?
         # Default to the non-hashed name in the specified output directory, which, for legacy
         # React on Rails, this is the output directory picked up by the asset pipeline.
         # For Shakapacker, this is the public output path defined in the (shaka/web)packer.yml file.

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 = Shakapacker # rubocop:disable Lint/ConstantDefinitionInBlock, RSpec/LeakyConstantDeclaration
+          Packer = ReactOnRails::PackerUtils.packer # rubocop:disable Lint/ConstantDefinitionInBlock, RSpec/LeakyConstantDeclaration
           allow(ReactOnRails::PackerUtils).to receive_messages(
             manifest_exists?: true,
             packer_public_output_path: generated_assets_full_path