Unlike with the InProcessRuntime, there is a two-phase initialization,
first when AgentsApp is built (when initial agents are registered) and
when it StartAsync()s and connects to the Gateway. Unfortunately, it is
possible to attempt to send direct RPC calls to the Gateway before the
message channel is opened; in this case, the Gateway has no connected
client corresponding to the RPC's clientId, and falls over.
The fix is to defer registering agents and subscriptions with the
gateway until after the connection is established after .StartAsync() is
called.
