test: remaining xUnit1051/1031/1030, CS8602/8604, CS0618, CS0219, CS0649, NU1510 fixes (lmxopcua#725)

Awaited deadline oracles get the runner token; sync-over-async tests made async;
nullable child models asserted via ShouldNotBeNull() return values; obsolete SDK
ctors replaced with the telemetry overloads; dead locals/fixture and the redundant
DI PackageReference removed. No suppressions.

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

tests/Client/ZB.MOM.WW.OtOpcUa.Client.UI.Tests/ReadWriteViewModelTests.cs

@@ -128,7 +128,7 @@ public class ReadWriteViewModelTests
         // because auto-read catches the exception too
         _vm.SelectedNodeId = "ns=2;s=SomeNode";
 
-        _vm.CurrentStatus.ShouldContain("Error");
+        _vm.CurrentStatus.ShouldNotBeNull().ShouldContain("Error");
     }
 
     /// <summary>Verifies that clear resets all properties.</summary>