Begin tracking a pending outbound request, keyed by (sessionToken, id).
Only a response that arrives on the SAME session token may resolve it, so
one session cannot satisfy another session's pending server->client request
even when both happen to use the same numeric id. The empty token registers
an unscoped (stateless / shared mode) waiter, resolvable only by a reply
that likewise carries the empty token.
Begin tracking a pending outbound request, keyed by (sessionToken, id). Only a response that arrives on the SAME session token may resolve it, so one session cannot satisfy another session's pending server->client request even when both happen to use the same numeric id. The empty token registers an unscoped (stateless / shared mode) waiter, resolvable only by a reply that likewise carries the empty token.