List Source Available Properties

List the properties a source's objects expose, so it can be narrowed to just the ones wanted.

Scoped to an existing API source; its stored token is used server-side to read the vendor's property catalogs, and the objects read are the ones the source stores. Source-scoped on purpose, exactly as custom-object discovery is: the default is to sync every property, so the create form never needs this picker — narrowing is a refinement on a source that already exists.

Takes no object parameter: the selection it feeds applies to every object the source syncs, so the result is their union, each property naming which of them expose it. A source whose objects have no property catalog reads as a 400 with the reason, not a 500.