Fix NoMethodError: call Engine.skip_version_validation? as class method

The initializer block context doesn't have direct access to class methods.
Changed from `skip_version_validation?` to `Engine.skip_version_validation?`
to properly call the class method.

Fixes error:
NoMethodError: undefined method `skip_version_validation?' for #<ReactOnRails::Engine>

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

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

Author: justin808

lib/react_on_rails/engine.rb

@@ -9,7 +9,7 @@ class Engine < ::Rails::Engine
     # Skip validation during installation tasks (e.g., shakapacker:install) or generator runtime
     initializer "react_on_rails.validate_version_and_package_compatibility" do
       config.after_initialize do
-        next if skip_version_validation?
+        next if Engine.skip_version_validation?
 
         Rails.logger.info "[React on Rails] Validating package version and compatibility..."
         VersionChecker.build.validate_version_and_package_compatibility!