tests fixes and some product advisories tunes ups

src/__Libraries/StellaOps.HybridLogicalClock/HlcTimestampJsonConverter.cs

@@ -23,7 +23,7 @@ public sealed class HlcTimestampJsonConverter : JsonConverter<HlcTimestamp>
     {
         if (reader.TokenType == JsonTokenType.Null)
         {
-            return default;
+            throw new JsonException("Cannot deserialize null to HlcTimestamp. Use HlcTimestamp? with NullableHlcTimestampJsonConverter for nullable timestamps.");
         }
 
         if (reader.TokenType != JsonTokenType.String)