Fix CI: Add using_swc? stub to JsDependencyManager spec

The JsDependencyManager tests create a mock test class that includes
the module. After adding the using_swc? call to add_js_dependencies,
the test class needs to provide this method (which comes from
GeneratorHelper in actual generator classes).

Changes:
- Add using_swc? method stub to test class (defaults to true)
- Add using_swc setter for test control
- Add test for SWC_DEPENDENCIES constant

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

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

Author: justin808

react_on_rails/spec/react_on_rails/generators/js_dependency_manager_spec.rb

@@ -26,6 +26,13 @@ def destination_root
         "/test/path"
       end
 
+      # Mock using_swc? from GeneratorHelper (defaults to true for SWC testing)
+      def using_swc?
+        @using_swc.nil? ? true : @using_swc
+      end
+
+      attr_writer :using_swc
+
       # Test helpers
       attr_writer :add_npm_dependencies_result
 
@@ -109,6 +116,12 @@ def errors
                                                                                            ])
     end
 
+    it "defines SWC_DEPENDENCIES" do
+      expect(ReactOnRails::Generators::JsDependencyManager::SWC_DEPENDENCIES).to(
+        eq(%w[@swc/core swc-loader])
+      )
+    end
+
     it "does not include Babel presets in REACT_DEPENDENCIES" do
       expect(ReactOnRails::Generators::JsDependencyManager::REACT_DEPENDENCIES).not_to include(
         "@babel/preset-react"