Fix: Properly access Pro configuration methods in ensure_webpack_generated_files_exists

The previous implementation used `respond_to?` to check for Pro-only methods
(rsc_bundle_js_file, react_client_manifest_file, react_server_client_manifest_file)
on the ReactOnRails::Configuration instance. However, these methods don't exist
on the base Configuration class - they only exist on ReactOnRailsPro.configuration.

This change follows the established pattern used in lib/react_on_rails/utils.rb:158-161
and lib/react_on_rails/packer_utils.rb:61-62 by:
1. Checking if Pro is available with ReactOnRails::Utils.react_on_rails_pro?
2. Accessing the Pro configuration object (ReactOnRailsPro.configuration)
3. Calling the Pro-specific methods on that object

This ensures the code works correctly in both scenarios:
- Open-source only: Pro check returns false, Pro files not added
- With Pro loaded: Pro files are correctly added to webpack_generated_files

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

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

Author: justin808

lib/react_on_rails/configuration.rb

@@ -361,9 +361,13 @@ def ensure_webpack_generated_files_exists
       return unless webpack_generated_files.empty?
 
       files = ["manifest.json", server_bundle_js_file]
-      files << rsc_bundle_js_file if respond_to?(:rsc_bundle_js_file)
-      files << react_client_manifest_file if respond_to?(:react_client_manifest_file)
-      files << react_server_client_manifest_file if respond_to?(:react_server_client_manifest_file)
+
+      if ReactOnRails::Utils.react_on_rails_pro?
+        pro_config = ReactOnRailsPro.configuration
+        files << pro_config.rsc_bundle_js_file
+        files << pro_config.react_client_manifest_file
+        files << pro_config.react_server_client_manifest_file
+      end
 
       self.webpack_generated_files = files.compact_blank
     end