Add missing `moderation_warning` notification support to grouped notifications API (#30576)

app/serializers/rest/notification_group_serializer.rb

@@ -11,6 +11,7 @@ class REST::NotificationGroupSerializer < ActiveModel::Serializer
   belongs_to :target_status, key: :status, if: :status_type?, serializer: REST::StatusSerializer
   belongs_to :report, if: :report_type?, serializer: REST::ReportSerializer
   belongs_to :account_relationship_severance_event, key: :event, if: :relationship_severance_event?, serializer: REST::AccountRelationshipSeveranceEventSerializer
+  belongs_to :account_warning, key: :moderation_warning, if: :moderation_warning_event?, serializer: REST::AccountWarningSerializer
 
   def status_type?
     [:favourite, :reblog, :status, :mention, :poll, :update].include?(object.type)
@@ -24,6 +25,10 @@ class REST::NotificationGroupSerializer < ActiveModel::Serializer
     object.type == :severed_relationships
   end
 
+  def moderation_warning_event?
+    object.type == :moderation_warning
+  end
+
   def page_min_id
     range = instance_options[:group_metadata][object.group_key]
     range.present? ? range[:min_id].to_s : object.notification.id.to_s