UpdateInbound restores the stored NodeID before the node-eligibility check, so
the payload can never introduce an assignment there — the check could only ever
fire on a row that already had one. A node's MTProto inbound arrives on the
master by adoption, which does not go through that check, so every later edit of
it was refused with "mtproto inbounds cannot be assigned to a node". That made
the share address of a node-managed MTProto inbound impossible to change from
the panel that generates its subscription links.
Refuse only a protocol change into an ineligible protocol, which is the one way
an update can still strand a row the master's sidecar loops would never
reconcile.
Closes#6415
Sanaei