Fix duplicate `@context` attribute in user export (#30653)

app/services/backup_service.rb

@@ -19,8 +19,8 @@ class BackupService < BaseService
 
   def build_outbox_json!(file)
     skeleton = serialize(collection_presenter, ActivityPub::CollectionSerializer)
-    skeleton[:@context] = full_context
-    skeleton[:orderedItems] = ['!PLACEHOLDER!']
+    skeleton['@context'] = full_context
+    skeleton['orderedItems'] = ['!PLACEHOLDER!']
     skeleton = Oj.dump(skeleton)
     prepend, append = skeleton.split('"!PLACEHOLDER!"')
     add_comma = false

spec/services/backup_service_spec.rb

@@ -55,9 +55,11 @@ RSpec.describe BackupService do
   end
 
   def expect_outbox_export
-    json = export_json(:outbox)
+    body = export_json_raw(:outbox)
+    json = Oj.load(body)
 
     aggregate_failures do
+      expect(body.scan('@context').count).to eq 1
       expect(json['@context']).to_not be_nil
       expect(json['type']).to eq 'OrderedCollection'
       expect(json['totalItems']).to eq 2
@@ -85,8 +87,12 @@ RSpec.describe BackupService do
     end
   end
 
+  def export_json_raw(type)
+    read_zip_file(backup, "#{type}.json")
+  end
+
   def export_json(type)
-    Oj.load(read_zip_file(backup, "#{type}.json"))
+    Oj.load(export_json_raw(type))
   end
 
   def include_create_item(status)