Sanity checks a provider assigned to the workspace. The agents of an on-premise provider, compared with those of every on-premise provider assigned to the workspace: agents sharing a machine, a memory per agent declared too low. A name shared with another provider the workspace sees, its own, shared or OctoPerf's, reported only when the caller can rename the provider: the result depends on the caller. 403 when the caller cannot read the provider, as for a public provider of another workspace; 404 when the provider is not assigned to the workspace