Serializers spec coverage (#24017)

.rubocop_todo.yml

spec/fabricators/encrypted_message_fabricator.rb

spec/serializers/activitypub/device_serializer_spec.rb

spec/serializers/activitypub/one_time_key_serializer_spec.rb

spec/serializers/activitypub/undo_like_serializer_spec.rb

spec/serializers/activitypub/vote_serializer_spec.rb

spec/serializers/rest/encrypted_message_serializer_spec.rb

spec/serializers/rest/instance_serializer_spec.rb

spec/serializers/rest/keys/claim_result_serializer_spec.rb

spec/serializers/rest/keys/device_serializer_spec.rb

spec/serializers/rest/keys/query_result_serializer_spec.rb

spec/serializers/rest/suggestion_serializer_spec.rb