Poll one generation task

Clerk-session only, and gated on a runId this caller owns. A FAILED generation is a 200 carrying an error field, NOT an HTTP error - an HTTP error here means the poll itself could not be performed, which is the distinction a client absorbing transient network failures depends on. Polling is not itself billed but is gated anyway, because an open poll would leak whether an arbitrary task id exists and what it produced.