fix(opcuaclient): review — UTC-kind the missing-time sentinel + test hardening

Code-review I2: CoerceDateTime's missing-field sentinel was DateTime.MinValue
(Kind=Unspecified) — a downstream .ToUniversalTime() could shift it; now UTC-kinded.
M4: assert BrowsePath namespace==0 + the sentinel's UTC Kind.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs

@@ -1751,8 +1751,12 @@ public sealed class OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWrit
         _ => value.ToString(),
     };
 
+    // Missing-field sentinel is UTC-kinded so a downstream .ToUniversalTime() can't shift it.
+    private static readonly DateTime MissingTimeSentinel =
+        DateTime.SpecifyKind(DateTime.MinValue, DateTimeKind.Utc);
+
     private static DateTime CoerceDateTime(object? value)
-        => value is DateTime dt ? dt : DateTime.MinValue;
+        => value is DateTime dt ? dt : MissingTimeSentinel;
 
     private static ushort CoerceSeverity(object? value)
     {