Refactor email template handling and error management

- Updated EmailTemplateDto to allow mutable Body and Subject properties.
- Implemented IRequest interface in UpdateEmailTemplateCommand for MediatR.
- Enhanced error handling in EmailTemplateController with NotFoundException.
- Introduced UpdateEmailTemplateCommandHandler for processing update commands.
- Added NotFoundException class for improved error handling.

EnvelopeGenerator.Application/DTOs/EmailTemplateDto.cs

@@ -22,11 +22,11 @@ namespace EnvelopeGenerator.Application.DTOs
         /// <summary>
         /// 
         /// </summary>
-        public required string Body { get; init; }
+        public required string Body { get; set; }
 
         /// <summary>
         /// 
         /// </summary>
-        public required string Subject { get; init; }
+        public required string Subject { get; set; }
     };
 }