Fix helpful message tests by mocking Shakapacker installation

Mock the Shakapacker binary existence check to return true, so that
ensure_shakapacker_installed skips the installation process and the
generator proceeds to show the success message that the tests expect.

This fixes the test failures where Shakapacker installation was failing
in CI environment, causing error messages instead of success messages.

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

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

Author: justin808

spec/react_on_rails/generators/install_generator_spec.rb

@@ -54,6 +54,13 @@
       GeneratorMessages.format_info(GeneratorMessages.helpful_message_after_installation)
     end
 
+    before do
+      # Mock Shakapacker installation to succeed so we get the success message
+      allow(File).to receive(:exist?).and_call_original
+      allow(File).to receive(:exist?).with("bin/shakapacker").and_return(true)
+      allow(File).to receive(:exist?).with("bin/shakapacker-dev-server").and_return(true)
+    end
+
     specify "base generator contains a helpful message" do
       run_generator_test_with_args(%w[], package_json: true)
       # GeneratorMessages.output is an array with the git error being the first one