Address code review feedback for validation skip documentation (#1926)

Implements three improvements based on code review feedback:

1. Enhanced documentation with concrete testing example
   - Added @example block showing how to clean up ENV in parallel tests
   - Makes the thread safety guidance more actionable for developers

2. Improved test assertion specificity
   - Added File.exist? spy assertion to verify short-circuit behavior
   - Confirms ENV check truly returns early without file system access

3. Clarified ensure block cleanup order importance
   - Added critical comment explaining why ENV cleanup must execute first
   - Prevents ENV pollution if print_generator_messages raises exception
   - Documents the intentional ordering for future maintainers

All tests pass and RuboCop reports no offenses.

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

Co-authored-by: Claude <noreply@anthropic.com>

Author: justin808

lib/generators/react_on_rails/install_generator.rb

@@ -74,6 +74,9 @@ def run_generators
         end
       ensure
         # Always clean up ENV variable, even if generator fails
+        # CRITICAL: ENV cleanup must come first to ensure it executes even if
+        # print_generator_messages raises an exception. This prevents ENV pollution
+        # that could affect subsequent processes.
         ENV.delete("REACT_ON_RAILS_SKIP_VALIDATION")
         print_generator_messages
       end

lib/react_on_rails/engine.rb

@@ -30,6 +30,12 @@ class Engine < ::Rails::Engine
     #   generators concurrently (e.g., in parallel tests), ensure tests run in separate
     #   processes to avoid ENV variable conflicts.
     #
+    # @example Testing with parallel processes
+    #   # In RSpec configuration:
+    #   config.before(:each) do |example|
+    #     ENV.delete("REACT_ON_RAILS_SKIP_VALIDATION")
+    #   end
+    #
     # @note Manual ENV Setting: While this ENV variable is designed to be set by generators,
     #   users can manually set it (e.g., `REACT_ON_RAILS_SKIP_VALIDATION=true rails server`)
     #   to bypass validation. This should only be done temporarily during debugging or

spec/react_on_rails/engine_spec.rb

@@ -50,6 +50,11 @@ module ReactOnRails
               expect(Rails.logger).not_to have_received(:debug)
                 .with("[React on Rails] Skipping validation during generator runtime")
             end
+
+            it "short-circuits before checking File.exist?" do
+              described_class.skip_version_validation?
+              expect(File).not_to have_received(:exist?)
+            end
           end
 
           context "when package.json is missing" do