Fix CodeRabbit review issues: invalid config, version consistency, missing require

Addresses 3 critical issues identified in PR #1896 review:

1. Remove invalid javascript_transpiler configuration
   - Shakapacker 9.3.0 does not support a top-level javascript_transpiler key
   - Removed lines 39-42 from shakapacker.yml template
   - Transpiler selection is configured via webpack config and installed loaders

2. Update Pro package.json files to Shakapacker 9.3.0
   - Updated react_on_rails_pro/spec/dummy/package.json (8.0.0 → 9.3.0)
   - Updated react_on_rails_pro/spec/execjs-compatible-dummy/package.json (8.0.0 → 9.3.0)
   - Added swc-loader and @swc/core dependencies to both files
   - Ensures version consistency across the codebase

3. Fix missing GeneratorMessages require in generator_helper.rb
   - Added require_relative "generator_messages" to prevent NameError
   - Issue occurred when detect_package_manager_and_exact_flag found multiple lock files
   - Now consistent with other generator files (base_generator.rb, install_generator.rb)

All RuboCop checks pass.

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

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

Author: justin808

lib/generators/react_on_rails/generator_helper.rb

@@ -2,6 +2,7 @@
 
 require "rainbow"
 require "json"
+require_relative "generator_messages"
 
 module GeneratorHelper
   def package_json

lib/generators/react_on_rails/templates/base/base/config/shakapacker.yml

@@ -36,11 +36,6 @@ default: &default
   # Reload manifest.json on all requests so we reload latest compiled packs
   cache_manifest: false
 
-  # Select JavaScript transpiler to use
-  # Available options: 'swc' (default, 20x faster), 'babel', or 'esbuild'
-  # Note: When using rspack, swc is used automatically regardless of this setting
-  javascript_transpiler: "swc"
-
   # Raises an error if there is a mismatch in the shakapacker gem and npm package being used
   ensure_consistent_versioning: true
 

react_on_rails_pro/spec/dummy/package.json

@@ -63,7 +63,9 @@
     "sass": "^1.43.4",
     "sass-loader": "^12.3.0",
     "sass-resources-loader": "^2.0.0",
-    "shakapacker": "8.0.0",
+    "shakapacker": "9.3.0",
+    "swc-loader": "^0.2.6",
+    "@swc/core": "^1.7.0",
     "style-loader": "^3.3.1",
     "tailwindcss": "^3.2.7",
     "terser-webpack-plugin": "5",

react_on_rails_pro/spec/execjs-compatible-dummy/package.json

@@ -30,7 +30,9 @@
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "react-on-rails": "link:.yalc/react-on-rails",
-    "shakapacker": "8.0.0",
+    "shakapacker": "9.3.0",
+    "swc-loader": "^0.2.6",
+    "@swc/core": "^1.7.0",
     "style-loader": "^4.0.0",
     "terser-webpack-plugin": "5",
     "webpack": "5",