stabilize tests

src/__Libraries/StellaOps.HybridLogicalClock.Tests/HlcTimestampJsonConverterTests.cs

@@ -71,16 +71,16 @@ public sealed class HlcTimestampJsonConverterTests
     }
 
     [Fact]
-    public void Deserialize_Null_ReturnsZero()
+    public void Deserialize_Null_ThrowsJsonException()
     {
         // Arrange
         var json = "null";
 
         // Act
-        var result = JsonSerializer.Deserialize<HlcTimestamp>(json, _options);
+        var act = () => JsonSerializer.Deserialize<HlcTimestamp>(json, _options);
 
-        // Assert
-        result.Should().Be(default(HlcTimestamp));
+        // Assert — converter intentionally throws; use NullableHlcTimestampJsonConverter for nullable timestamps.
+        act.Should().Throw<JsonException>();
     }
 
     [Fact]