refactor(host)/test: M2.14 review nits — simplify probe cancellation + pre-cancelled-token test (#28)

- Remove redundant linked CancellationTokenSource in ProbeAsync; pass the
  framework cancellationToken and ProbeTimeout directly to Ask (the two-CTS
  pattern was redundant — Ask already honours both the timeout and the token).
- Add EchoActor XML <remarks> explaining why no Receive<Identify> handler is
  needed (ActorBase answers Identify automatically).
- Add PreCancelledToken_ReportsUnhealthy_DoesNotThrow test: verifies the
  never-throws guarantee on the shutdown-race path (token already cancelled
  before CheckHealthAsync is invoked).

src/ZB.MOM.WW.ScadaBridge.Host/Health/RequiredSingletonsHealthCheck.cs

@@ -157,11 +157,9 @@ public sealed class RequiredSingletonsHealthCheck : IHealthCheck
             // ActorSelection so a missing path resolves an ActorIdentity with a null
             // Subject (rather than throwing) within the bounded timeout.
             var selection = system.ActorSelection($"/user/{proxyName}");
-            using var cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
-            cts.CancelAfter(ProbeTimeout);
 
             var identity = await selection
-                .Ask<ActorIdentity>(new Identify(proxyName), ProbeTimeout, cts.Token)
+                .Ask<ActorIdentity>(new Identify(proxyName), ProbeTimeout, cancellationToken)
                 .ConfigureAwait(false);
 
             return (proxyName, identity.Subject is not null);