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'

EnvelopeGenerator.Application/Envelopes/Commands/CreateEnvelopeCommandHandler.cs

@@ -302,7 +302,7 @@ public class CreateEnvelopeCommandHandler : IRequestHandler<CreateEnvelopeComman
             ExpiresWarningWhen: expiresWarningWhenDays > 0 ? DateTime.Now.AddDays(expiresWarningWhenDays) : null,
             ContractType: existing?.ContractType
                 ?? selectedType?.ContractType
-                ?? (int)ContractType.Contract);
+                ?? ContractType.Contract);
     }
 
     private static string ResolveLanguage(string? preferred, string? fallback1, string? fallback2)
@@ -333,7 +333,7 @@ public class CreateEnvelopeCommandHandler : IRequestHandler<CreateEnvelopeComman
         int ExpiresWarningWhenDays,
         DateTime? ExpiresWhen,
         DateTime? ExpiresWarningWhen,
-        int ContractType);
+        ContractType ContractType);
 
     private async Task<string> ResolveSenderReferenceAsync(Envelope envelope, CancellationToken cancel)
     {