List organization inference gateway providers

Defaults to scope=usable: returns active providers granted to the caller through active model groups and credential sets, with usable model aliases and any member authorization requests. A granted provider can remain discoverable with no usable models. scope=manageable requires the View Gateway providers permission and enabled Gateway management, and returns provider details including disabled providers; credential secrets are never returned, and people's names and emails only to callers who also hold Manage Gateway providers.