DGS-21279 Ensure unscaled values are positive in decimal conversions (#2490)

* DGS-21279 Ensure unscaled values are positive in decimal conversions

* Minor cleanup

Author: rayokota

src/Confluent.SchemaRegistry.Serdes.Protobuf/DecimalExtensions.cs

@@ -35,8 +35,10 @@ public static Decimal ToProtobufDecimal(this decimal value)
         {
             var bytes = GetBytesFromDecimal(value);
 
-            var unscaledValueBytes = new byte[12];
-            Array.Copy(bytes, unscaledValueBytes, unscaledValueBytes.Length);
+            // Copy the 12 bytes into an array of size 13 so that the last byte is 0,
+            // which will ensure that the unscaled value is positive.
+            var unscaledValueBytes = new byte[13];
+            Array.Copy(bytes, unscaledValueBytes, 12);
 
             var unscaledValue = new BigInteger(unscaledValueBytes);
             var scale = bytes[14];

test/Confluent.SchemaRegistry.Serdes.UnitTests/ProtoDecimal.cs

@@ -19,7 +19,9 @@ public void ConvertDecimals()
             -123456789123456789.56m,
             -1234m,
             -1234.5m,
-            -1234.56m
+            -1234.56m,
+            4.1748330066797328106875724500m,
+            -4.1748330066797328106875724500m
         };
 
         foreach (var input in inputs)