Fix CI failure by not setting default generated_assets_dir with Shakapacker

The configuration was automatically setting generated_assets_dir to a default
value that didn't match Shakapacker's public_output_path, causing validation
errors in CI and example apps.

Since Shakapacker manages its own public_output_path configuration, we should
not set a default generated_assets_dir when using Shakapacker. This prevents
path mismatches and allows proper Shakapacker configuration.

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

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

Author: justin808

lib/react_on_rails/configuration.rb

@@ -269,8 +269,10 @@ def check_server_render_method_is_only_execjs
     def ensure_generated_assets_dir_present
       return if generated_assets_dir.present?
 
-      self.generated_assets_dir = DEFAULT_GENERATED_ASSETS_DIR
-      Rails.logger.warn "ReactOnRails: Set generated_assets_dir to default: #{DEFAULT_GENERATED_ASSETS_DIR}"
+      # When using Shakapacker, don't set a default generated_assets_dir since
+      # Shakapacker manages its own public_output_path configuration
+      # This prevents configuration mismatches between ReactOnRails and Shakapacker
+      Rails.logger.warn "ReactOnRails: No generated_assets_dir specified, using Shakapacker public_output_path"
     end
 
     def configure_generated_assets_dirs_deprecation

spec/react_on_rails/configuration_spec.rb

@@ -313,6 +313,19 @@ module ReactOnRails
       expect(ReactOnRails.configuration.random_dom_id).to be(false)
     end
 
+    it "works without specifying generated_assets_dir when using Shakapacker" do
+      allow(ReactOnRails::PackerUtils).to receive_message_chain("packer.config.public_output_path")
+        .and_return(Pathname.new("/tmp/public/packs"))
+
+      expect do
+        ReactOnRails.configure do |config|
+          config.server_bundle_js_file = "server.js"
+        end
+      end.not_to raise_error
+
+      expect(ReactOnRails.configuration.generated_assets_dir).to be_blank
+    end
+
     it "calls raise_missing_components_subdirectory if auto_load_bundle = true & components_subdirectory is not set" do
       expect do
         ReactOnRails.configure do |config|