Check whether an AI-designed slide is ready and see its preview

The status link from POST /v1/slideshows/{id}/slides/ai: status is Processing, Ready (see previews; insert with insertUrl), Inserted (insertedSlideIds), NeedsClarification (read question to the person, then start a new job with the answer in prompt) or Failed (error, and errorCode when there is one). Ready with an error means adding the slides stopped part-way (e.g. storage full): insertedSlideIds lists what was added, and the insert call adds the rest without doubling any. A job the service could not finish (a restart) turns Failed; start it again. Jobs are kept for 24 hours.