fix: Add missing 'add' method to PackageManager mock in test

The test was failing because the mock PackageManager double only
stubbed the 'install' method, but the actual code calls
'pj.manager.add(packages_array)' in add_js_dependency.

This commit adds the 'add' method to the mock's allowed methods,
allowing the test to properly verify that package_json gem methods
are used instead of system() commands for dependency installation.

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

spec/react_on_rails/generators/message_deduplication_spec.rb

@@ -87,7 +87,7 @@
         # add_npm_dependencies is from GeneratorHelper and is used by add_js_dependencies_batch
         # Mock package_json to prevent actual package manager calls
         # rubocop:disable RSpec/VerifiedDoubles
-        mock_manager = double("PackageManager", install: true)
+        mock_manager = double("PackageManager", install: true, add: true)
         mock_package_json = double("PackageJson", manager: mock_manager)
         # rubocop:enable RSpec/VerifiedDoubles
         allow(install_generator).to receive_messages(