Fix default variable value serialization issue

src/modules/Elsa.Workflows.Core/Serialization/Converters/JsonPrimitiveToStringConverter.cs

@@ -0,0 +1,43 @@
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Elsa.Workflows.Core.Serialization.Converters;
+
+/// <summary>
+/// Converts primitives to and from JSON strings.
+/// </summary>
+public class JsonPrimitiveToStringConverter : JsonConverter<string>
+{
+    /// <inheritdoc />
+    public override string Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        switch (reader.TokenType)
+        {
+            case JsonTokenType.True:
+                return "True";
+            case JsonTokenType.False:
+                return "False";
+            case JsonTokenType.Number when reader.TryGetInt64(out var l):
+                return l.ToString();
+            case JsonTokenType.Number:
+                return reader.GetDouble().ToString(CultureInfo.InvariantCulture);
+            case JsonTokenType.String when reader.TryGetDateTimeOffset(out var datetime):
+                return datetime.ToString();
+            case JsonTokenType.String:
+                return reader.GetString()!;
+            default:
+            {
+                // Use JsonElement as fallback.
+                using var document = JsonDocument.ParseValue(ref reader);
+                return document.RootElement.Clone().ToString();
+            }
+        }
+    }
+
+    /// <inheritdoc />
+    public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
+    {
+        writer.WriteStringValue(value);
+    }
+}

src/modules/Elsa.Workflows.Core/Serialization/Converters/VariableConverter.cs

@@ -27,7 +27,9 @@ public class VariableConverter : JsonConverter<Variable>
     /// <inheritdoc />
     public override Variable? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
     {
-        var model = JsonSerializer.Deserialize<VariableModel>(ref reader, options)!;
+        var newOptions = new JsonSerializerOptions(options);
+        newOptions.Converters.Add(new JsonPrimitiveToStringConverter());
+        var model = JsonSerializer.Deserialize<VariableModel>(ref reader, newOptions)!;
         var variable = Map(model);
 
         return variable;