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

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

lib/chatwoot_app.rb

@@ -17,6 +17,10 @@ module ChatwootApp
     @enterprise ||= root.join('enterprise').exist?
   end
 
+  def self.chatwoot_cloud?
+    enterprise? && GlobalConfig.get_value('DEPLOYMENT_ENV') == 'cloud'
+  end
+
   def self.custom?
     @custom ||= root.join('custom').exist?
   end