Refactor ContractType property to use enum

Updated the `ContractType` property in the `EnvelopeTypeDto` class from a nullable integer (`int?`) to a nullable `ContractType` enum (`ContractType?`). This change enhances type safety and clarifies the expected values for the property.

EnvelopeGenerator.Application/Common/Dto/EnvelopeTypeDto.cs

@@ -80,5 +80,5 @@ public class EnvelopeTypeDto
     /// <summary>
     /// Gets or sets the contract type associated with the envelope type.
     /// </summary>
-    public int? ContractType { get; set; }
+    public ContractType? ContractType { get; set; }
 }