fix: Remove obsolete using_packer? method references

- Remove call to undefined ReactOnRails::PackerUtils.using_packer? method
- Update test mocks to use Shakapacker directly instead of packer abstraction
- Fix Shakapacker::Manifest::MissingEntryError reference in tests

The using_packer? method was removed in a recent refactor that eliminated
the unnecessary packer abstraction layer. This commit aligns the code with
those changes.

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

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

Author: github-actions[bot]

lib/react_on_rails/utils.rb

@@ -86,9 +86,6 @@ def self.bundle_js_file_path(bundle_name)
     end
 
     private_class_method def self.bundle_js_file_path_with_packer(bundle_name)
-      # Handle test scenarios where packer is mocked as unavailable
-      return File.join(generated_assets_full_path, bundle_name) unless ReactOnRails::PackerUtils.using_packer?
-
       is_server_bundle = server_bundle?(bundle_name)
       config = ReactOnRails.configuration
       root_path = Rails.root || "."

spec/react_on_rails/test_helper/webpack_assets_status_checker_spec.rb

@@ -62,7 +62,6 @@
         let(:fixture_dirname) { "assets_with_manifest_exist_server_bundle_separate" }
 
         before do
-          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
@@ -73,7 +72,7 @@
             .and_return(File.join(generated_assets_full_path, "manifest.json"))
           allow(ReactOnRails::PackerUtils).to receive(:bundle_js_uri_from_packer)
             .with("server-bundle.js")
-            .and_raise(Packer::Manifest::MissingEntryError)
+            .and_raise(Shakapacker::Manifest::MissingEntryError)
           touch_files_in_dir(generated_assets_full_path)
         end
 

spec/react_on_rails/utils_spec.rb

@@ -256,7 +256,7 @@ def mock_dev_server_running
 
                 # Since server_bundle_output_path is configured, should return path immediately
                 # without trying manifest lookup
-                expect(ReactOnRails::PackerUtils.packer).not_to receive(:manifest)
+                expect(::Shakapacker).not_to receive(:manifest)
                 expect(File).not_to receive(:exist?)
 
                 path = described_class.server_bundle_js_file_path