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>

enterprise/app/fields/account_features_field.rb

@@ -0,0 +1,7 @@
+require 'administrate/field/base'
+
+class AccountFeaturesField < Administrate::Field::Base
+  def to_s
+    data
+  end
+end