Get Next Autoplay Item
POST/mw/properties/:propertyId/autoplay/next
Given the media item the caller is currently on, returns the remaining run of items in a sidebar tab or group marked as sequential content. Returns an empty contents array, not an error, when there's nothing left to play (media_id not in a sequential scope, or already at the end of its run). Does not filter on the in-situ hierarchical permissions -- unauthorized items are returned as-is for the client to gate. (This is why we return N items instead of 1.)