Decline a message request

The caller says no. Their seat stays, marked declined, and they are "as if left": out of every list, delivery, broadcast, push, activity and presence, and never sees the chat; the person who wrote is not told: the caller stays in the member list they read, and they keep seeing delivered_as "request". Asking again (a new 1:1 from them, a new_session, adding them to the group again) silently reuses this chat and reaches nobody, until the caller acts: opens a 1:1 with them, or writes in the chat, and the seat is ordinary again. Idempotent: 204 again. A chat the caller has no seat in is the same 404 as an unknown id; a chat that is not a request is refused (422 not_a_request) and left alone. Blocking someone declines every pending request from them.