Address edge cases in validation and error handling

.gitignore validation improvements:
- Check multiple unique markers (Lefthook, Testing, Playwright)
- Prevents false positives from partial content matches
- More robust detection of duplicate entries

Port polling improvements:
- Fail fast in CI environments when port unavailable
- Prevents flaky tests from port conflicts in CI
- Local development continues with warning
- CI_ENV detection makes behavior environment-aware

These changes improve reliability in CI while maintaining
flexibility for local development.

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

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

Author: justin808

packages/shakacode_demo_common/lib/generators/shakacode_demo_common/install/install_generator.rb

@@ -170,7 +170,14 @@ def display_post_install
       private
 
       def gitignore_contains_our_content?
-        File.exist?('.gitignore') && File.read('.gitignore').include?('# Lefthook')
+        return false unless File.exist?('.gitignore')
+
+        content = File.read('.gitignore')
+        # Check for multiple unique markers to ensure our full content is present
+        # This prevents false positives if only partial content exists
+        content.include?('# Lefthook') &&
+          content.include?('# Testing') &&
+          content.include?('# Playwright')
       end
 
       def gem_root_path

packages/shakacode_demo_common/lib/shakacode_demo_common/e2e_test_runner.rb

@@ -79,7 +79,12 @@ def run_playwright_tests(env)
 
     def cleanup_between_modes
       puts 'Waiting for server to release port...'
-      port_available? || puts('Warning: Continuing despite port possibly being in use')
+      return if port_available?
+
+      # Port is still in use after timeout - fail fast in CI to prevent flaky tests
+      raise 'Port still in use after timeout. Failing to prevent flaky tests in CI.' if ENV['CI']
+
+      puts 'Warning: Continuing despite port possibly being in use'
     end
 
     # Checks if port becomes available within timeout period