`as: "json"` is a @rails/request.js option; plain fetch ignores it and
sends `Accept: */*`, so Autocompletable::UsersController answered with
its HTML format and response.json() threw. The new ping form never
showed any suggestions. Send an explicit Accept header instead.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Sam Ruby