chore: improve plan-based feature handling with plan hierarchy (#11335)

- Refactor HandleStripeEventService to better manage features by plan
- Add constants for features available in each plan tier (Startup,
Business, Enterprise)
- Add channel_instagram to Startup plan features
- Improve downgrade handling to properly disable higher-tier features
- Clean up and optimize tests for maintainability
- Add comprehensive test coverage for plan upgrades and downgrades

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/helpers/super_admin/account_features_helper.rb

@@ -15,7 +15,7 @@ module SuperAdmin::AccountFeaturesHelper
   end
 
   def self.filter_internal_features(features)
-    return features if GlobalConfig.get_value('DEPLOYMENT_ENV') == 'cloud'
+    return features if ChatwootApp.chatwoot_cloud?
 
     internal_features = account_features.select { |f| f['chatwoot_internal'] }.pluck('name')
     features.except(*internal_features)