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/ModbusDriverProbeParityTests.cs

@@ -51,6 +51,6 @@ public sealed class ModbusDriverProbeParityTests
         var (target, error) = ModbusDriverProbe.TryParseProbeTarget(
             "not valid json {{", fallbackTimeout: TimeSpan.FromSeconds(5));
         target.ShouldBeNull();
-        error.ShouldContain("invalid");
+        error.ShouldNotBeNull().ShouldContain("invalid");
     }
 }