Enhance email template handling and documentation

- Added XML documentation to the `Handle` method in `UpdateEmailTemplateCommandHandler`.
- Improved readability in `ReadEmailTemplateQueryHandler` by storing the mapped response in a variable.
- Updated properties in `ReadEmailTemplateResponse` to be mutable and renamed `Type` to `Name` with a type change from `int` to `string`.
- Added data annotations in `EmailTemplate` for `AddedWhen` and introduced a new nullable `ChangedWhen` property.
- Included necessary using directives for data annotations in `EmailTemplate.cs`.

EnvelopeGenerator.Application/EmailTemplates/Commands/Update/UpdateEmailTemplateCommandHandler.cs

@@ -24,6 +24,14 @@ public class UpdateEmailTemplateCommandHandler : IRequestHandler<UpdateEmailTemp
         _repository = repository;
     }
 
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancel"></param>
+    /// <returns></returns>
+    /// <exception cref="InvalidOperationException"></exception>
+    /// <exception cref="NotFoundException"></exception>
     public async Task Handle(UpdateEmailTemplateCommand request, CancellationToken cancel)
     {
         var temp = (request.EmailTemplateQuery?.Id is int id