List extension namespaces for an item

Returns every extension namespace attached to the item that the caller has permission to read. Extensions are namespaced JSON sidecars (<app>.*, <publisher>.*, user.*) — credentials must declare each namespace in their extension_permissions map; namespaces outside that map are silently filtered.

For list views, prefer GET /items?include=extensions to hydrate extensions inline across a page with the same permission rule applied. See Extensions.