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/models/enterprise/account.rb

@@ -1,4 +1,8 @@
 module Enterprise::Account
+  # TODO: Remove this when we upgrade administrate gem to the latest version
+  # this is a temporary method since current administrate doesn't support virtual attributes
+  def manually_managed_features; end
+
   def mark_for_deletion(reason = 'manual_deletion')
     result = custom_attributes.merge!('marked_for_deletion_at' => 7.days.from_now.iso8601, 'marked_for_deletion_reason' => reason) && save