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/Queries/Read/ReadEmailTemplateQueryHandler.cs

@@ -45,6 +45,8 @@ public class ReadEmailTemplateQueryHandler : IRequestHandler<ReadEmailTemplateQu
                 ? await _repository.ReadByNameAsync(type)
                 : throw new InvalidOperationException("Either a valid integer ID or a valid EmailTemplateType must be provided in the request.");
 
-        return temp is null ? null : _mapper.Map<ReadEmailTemplateResponse>(temp);
+        var res = _mapper.Map<ReadEmailTemplateResponse>(temp);
+
+        return res;
     }
 }