test(virtual-tags): re-apply until the host has pruned the dead child (lmxopcua#769)

Child_is_respawned_after_unexpected_termination watched the child die, then re-applied once. The host prunes _children only when its own Terminated arrives, so a re-apply that lands first still sees vt-1 as present, spawns nothing, and the test times out at 15 s (caught 1 in 5 full Runtime.Tests runs on 2026-11-09). Re-apply inside AwaitAssert until the new RegisterInterest arrives; the assertions are unchanged. 30/30 isolated runs pass.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/VirtualTags/VirtualTagHostActorTests.cs

@@ -147,9 +147,16 @@ public sealed class VirtualTagHostActorTests : RuntimeActorTestBase
         mux.ExpectMsg<DependencyMuxActor.UnregisterInterest>(TimeSpan.FromSeconds(5));
 
         // Re-apply with the same plan — host should see vt-1 absent from _children and spawn fresh.
-        host.Tell(new VirtualTagHostActor.ApplyVirtualTags(plan));
-        var reg2 = mux.ExpectMsg<DependencyMuxActor.RegisterInterest>(TimeSpan.FromSeconds(5));
-        reg2.TagRefs.ShouldContain("a");
+        // The test saw the child die, but the host's own Terminated (which prunes _children) can land
+        // after a single re-apply, which then still sees vt-1 as present and spawns nothing
+        // (lmxopcua#769: this raced ~1 in 6 full runs). Re-apply until the host has pruned it.
+        DependencyMuxActor.RegisterInterest? reg2 = null;
+        AwaitAssert(() =>
+        {
+            host.Tell(new VirtualTagHostActor.ApplyVirtualTags(plan));
+            reg2 = mux.ExpectMsg<DependencyMuxActor.RegisterInterest>(TimeSpan.FromMilliseconds(500));
+        }, TimeSpan.FromSeconds(10), TimeSpan.FromMilliseconds(50));
+        reg2!.TagRefs.ShouldContain("a");
 
         // The new child must be a different actor ref than the one we killed.
         var secondChild = mux.LastSender;