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>

enterprise/app/fields/manually_managed_features_field.rb

@@ -0,0 +1,31 @@
+require 'administrate/field/base'
+
+class ManuallyManagedFeaturesField < Administrate::Field::Base
+  def data
+    Internal::Accounts::InternalAttributesService.new(resource).manually_managed_features
+  end
+
+  def to_s
+    data.is_a?(Array) ? data.join(', ') : '[]'
+  end
+
+  def all_features
+    # Business and Enterprise plan features only
+    Enterprise::Billing::HandleStripeEventService::BUSINESS_PLAN_FEATURES +
+      Enterprise::Billing::HandleStripeEventService::ENTERPRISE_PLAN_FEATURES
+  end
+
+  def selected_features
+    # If we have direct array data, use it (for rendering after form submission)
+    return data if data.is_a?(Array)
+
+    # Otherwise, use the service to retrieve the data from internal_attributes
+    if resource.respond_to?(:internal_attributes)
+      service = Internal::Accounts::InternalAttributesService.new(resource)
+      return service.manually_managed_features
+    end
+
+    # Fallback to empty array if no data available
+    []
+  end
+end