When the switch forwards a blinded hop identified by node ID, it has not
yet resolved a concrete outgoing channel at interception time. Expose the
next hop to the interceptor: InterceptedForward.Packet() reports the
packet's outgoing channel as-is (hop.Exit, since none is selected yet) and
carries the requested pubkey in OutgoingNodeID.
At the RPC boundary, forwardInterceptor.onIntercept maps a node-ID hop to
the reserved NodeIDForwardSCID sentinel in outgoing_requested_chan_id and
the pubkey in outgoing_requested_node_id, so a client switching on a zero
channel ID to detect the exit hop does not misread the forward as a final
receive. The sentinel is a wire-only concern, applied where the request is
built rather than in the switch's internal InterceptedPacket, which stays
truthful (OutgoingNodeID.IsSome() is the node-ID discriminator).
(cherry picked from commit 32373b76c7)
fa55f44367