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.Domain/Entities/Envelope.cs

@@ -83,7 +83,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public string Comment { get; set; }
 
         [Column("CONTRACT_TYPE")]
-        public int? ContractType { get; set; }
+        public ContractType? ContractType { get; set; }
 
 #if NETFRAMEWORK
         [NotMapped]