Fix BackoffSupervisorSpec final stop message test race condition (#8032)

The test was flaky because:
1. It killed the child, waited for supervisor to restart it
2. Then sent the final stop message
3. Expected supervisor to terminate

But the supervisor only terminates when the child stops AFTER the
final stop message was received. The test needed to kill the child
after sending the stop message.

Fix: Wait for the restarted child (c2), send the stop message,
then kill c2. This matches the expected behavior where the supervisor
terminates because it received the final stop flag before the child
stopped.

Author: Aaronontheweb

src/core/Akka.Tests/Pattern/BackoffSupervisorSpec.cs

@@ -499,12 +499,31 @@ public async Task BackoffSupervisor_must_not_stop_when_final_stop_message_has_no
             await WatchAsync(c1);
             await supervisorWatcher.WatchAsync(supervisor);
 
+            // Kill child without sending final stop message first
             c1.Tell(PoisonPill.Instance);
             await ExpectTerminatedAsync(c1);
-            supervisor.Tell("ping");
-            await supervisorWatcher.ExpectNoMsgAsync(TimeSpan.FromMilliseconds(20)); // supervisor must not terminate
 
+            // Wait for the supervisor to restart the child (backoff delay is 100ms minimum)
+            // This verifies that supervisor didn't stop - it restarted the child instead
+            IActorRef c2 = null;
+            await AwaitAssertAsync(async () =>
+            {
+                supervisor.Tell(BackoffSupervisor.GetCurrentChild.Instance);
+                c2 = (await ExpectMsgAsync<BackoffSupervisor.CurrentChild>()).Ref;
+                c2.Should().NotBeSameAs(c1);
+                c2.IsNobody().Should().BeFalse();
+            });
+
+            // Supervisor should still be alive (no final stop message received yet)
+            await supervisorWatcher.ExpectNoMsgAsync(TimeSpan.FromMilliseconds(100));
+
+            // Now send the final stop message and kill the new child
+            // The supervisor should terminate because it received the final stop message
+            await WatchAsync(c2);
             supervisor.Tell(stopMessage);
+            await ExpectMsgAsync(stopMessage); // Child echoes the message
+            c2.Tell(PoisonPill.Instance);
+            await ExpectTerminatedAsync(c2);
             await supervisorWatcher.ExpectTerminatedAsync(supervisor);
         }
     }