Refactor ContractType to use enum

Changed the `ContractType` property in `EnvelopeDto`, `CreateEnvelopeCommandHandler`, and `Envelope` classes from a nullable `int` to a nullable `ContractType` enum. This improves type safety and consistency across the codebase by using a specific enum type instead of a generic integer. Updated logic in `CreateEnvelopeCommandHandler` to remove unnecessary casting.

EnvelopeGenerator.Application/Common/Dto/EnvelopeDto.cs

@@ -62,7 +62,7 @@ public record EnvelopeDto : IEnvelope
     /// <summary>
     /// 
     /// </summary>
-    public int? ContractType { get; set; }
+    public ContractType? ContractType { get; set; }
 
     /// <summary>
     /// Default value is 'de-DE'