Wait for the device's next result or a status change (server-held, no polling)

The session's own agent only (anyone else: the same 404 as getDeviceSession). ONE request that Salt holds open up to timeout seconds and answers the moment a device-authored message newer than after exists, or the session's status differs from status, or the session has ended; already true means answered at once. Otherwise it answers timed_out: true with empty messages. Do not loop on it faster than it answers: a hold is a server thread, so each agent may hold 2 and each process a few; past a cap you are answered immediately with retry_after_ms and no hold (wait that long, then ask again). after omitted: only status changes and device messages that arrive after the request wake it. Messages are ciphertext in the shape GET /chats/{id} returns, oldest first, at most 20; ask again with after = the last id for more.