Add media attachment description limit to instance API response (#33153)

spec/requests/api/v2/instance_spec.rb

@@ -55,6 +55,9 @@ RSpec.describe 'Instances' do
             max_characters: StatusLengthValidator::MAX_CHARS,
             max_media_attachments: Status::MEDIA_ATTACHMENTS_LIMIT
           ),
+          media_attachments: include(
+            description_limit: MediaAttachment::MAX_DESCRIPTION_LENGTH
+          ),
           polls: include(
             max_options: PollValidator::MAX_OPTIONS
           )