Poll Source Oauth Grant

Collect the outcome of an OAuth connect flow the caller started, by its state.

This is the reliable delivery path for the oauth_grant_id: the callback's postMessage is best-effort, and providers whose consent page sets Cross-Origin-Opener-Policy sever the popup's window.opener so it never lands — so poll this instead. Scoped to the tenant and user that started the flow, and single-use. Returns {"oauth_grant_id": ...} once ready, {"error": ...} for a recorded failure, or {"status": "pending"} while the callback is still in flight (keep polling). Inert (always pending) for a state that isn't the caller's live flow.