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/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusLifecycleHygieneTests.cs

@@ -497,8 +497,8 @@ public sealed class ModbusLifecycleHygieneTests(ITestOutputHelper output)
         });
 
         await Task.WhenAll(
-            Task.Run(() => drv.InitializeAsync("{}", CancellationToken.None)),
-            Task.Run(() => drv.InitializeAsync("{}", CancellationToken.None)));
+            Task.Run(() => drv.InitializeAsync("{}", CancellationToken.None), TestContext.Current.CancellationToken),
+            Task.Run(() => drv.InitializeAsync("{}", CancellationToken.None), TestContext.Current.CancellationToken));
 
         var built = handed.ToArray();
         built.Length.ShouldBe(2);