Skip version validation when package.json doesn't exist

The version checker was failing during `rake shakapacker:install` because
it runs before package.json is created. This broke the generator examples
in CI with error: "package.json file not found".

This commit updates the engine initializer to skip validation when
package.json doesn't exist yet, allowing installation tasks to complete
successfully.

The validation will still run normally once package.json exists, ensuring
version compatibility in production apps.

Fixes generator test failures:
- examples (3.2, minimum)
- examples (3.4, latest)

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

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

Author: justin808

lib/react_on_rails/engine.rb

@@ -6,8 +6,13 @@ module ReactOnRails
   class Engine < ::Rails::Engine
     # Validate package versions and compatibility on Rails startup
     # This ensures the application fails fast if versions don't match or packages are misconfigured
+    # Skip validation during installation tasks (e.g., shakapacker:install)
     initializer "react_on_rails.validate_version_and_package_compatibility" do
       config.after_initialize do
+        # Skip validation if package.json doesn't exist yet (during initial setup)
+        package_json = VersionChecker::NodePackageVersion.package_json_path
+        next unless File.exist?(package_json)
+
         Rails.logger.info "[React on Rails] Validating package version and compatibility..."
         VersionChecker.build.validate_version_and_package_compatibility!
         Rails.logger.info "[React on Rails] Package validation successful"