Ensure that JsonHelper.FindChangedFields does not return BigInteger

Big integer is not supported in json becuase it serialized on the usless form
{"IsPowerOfTwo":true,"IsZero":false,"IsOne":false,"IsEven":true,"Sign":1}

src/Altinn.App.Core/Helpers/JsonHelper.cs

@@ -1,5 +1,6 @@
 #nullable enable
 
+using System.Numerics;
 using Altinn.App.Core.Features;
 using Altinn.Platform.Storage.Interface.Models;
 using Microsoft.Extensions.Logging;
@@ -182,7 +183,13 @@ private static void FindDiff(Dictionary<string, object?> dict, JToken? old, JTok
                     break;
 
                 default:
-                    dict.Add(prefix, current == null ? null : ((JValue)current).Value);
+                    var convertedValue = (current as JValue)?.Value switch
+                    {
+                        // BigInteger is not supported in json, so try to reduce to decimal, if possible, or string if too big
+                        BigInteger bigInt => bigInt <= new BigInteger(decimal.MaxValue) ? (decimal)bigInt : bigInt.ToString(System.Globalization.NumberFormatInfo.InvariantInfo),
+                        _ => (current as JValue)?.Value
+                    };
+                    dict.Add(prefix, convertedValue);
                     break;
             }
         }