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.

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientHistoryTests.cs

@@ -112,6 +112,7 @@ public sealed class OpcUaClientHistoryTests
             clause.AttributeId.ShouldBe(Attributes.Value);
             clause.BrowsePath.Count.ShouldBe(1);
             clause.BrowsePath[0].Name.ShouldBe(expected[i]);
+            clause.BrowsePath[0].NamespaceIndex.ShouldBe((ushort)0); // BaseEventType fields live in ns 0
         }
     }
 
@@ -165,6 +166,7 @@ public sealed class OpcUaClientHistoryTests
         mapped[0].EventId.ShouldBe(Convert.ToBase64String(new byte[] { 9 }));
         mapped[0].SourceName.ShouldBeNull();
         mapped[0].EventTimeUtc.ShouldBe(DateTime.MinValue);
+        mapped[0].EventTimeUtc.Kind.ShouldBe(DateTimeKind.Utc); // sentinel is UTC-kinded, not Unspecified
         mapped[0].Severity.ShouldBe((ushort)0);
     }
 }