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.GeneratorAPI/Controllers/EmailTemplateController.cs

@@ -10,6 +10,7 @@ using EnvelopeGenerator.Application.DTOs;
 using MediatR;
 using System.Threading.Tasks;
 using DigitalData.UserManager.Application.Services;
+using EnvelopeGenerator.Application.Exceptions;
 
 namespace EnvelopeGenerator.GeneratorAPI.Controllers;
 
@@ -116,11 +117,16 @@ public class EmailTemplateController : ControllerBase
             }
             else
             {
-                update.EmailTemplateQuery = email;
-                // Logic for updating the email template
+                var reset = _mapper.Map<ResetEnvelopeTemplateCommand>(email);
+                await _mediator.Send(update);
+                return Ok();
             }
 
         }
+        catch(NotFoundException)
+        {
+            return BadRequest();
+        }
         catch (Exception ex)
         {
             _logger.LogError(ex, "An unexpected error occurred. {message}", ex.Message);