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/EmailTemplates/Commands/Update/UpdateEmailTemplateCommandHandler.cs

@@ -0,0 +1,43 @@
+using DigitalData.Core.Abstractions.Infrastructure;
+using EnvelopeGenerator.Application.DTOs;
+using EnvelopeGenerator.Application.Exceptions;
+using EnvelopeGenerator.Domain.Entities;
+using MediatR;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+
+namespace EnvelopeGenerator.Application.EmailTemplates.Commands.Update;
+
+/// <summary>
+/// 
+/// </summary>
+public class UpdateEmailTemplateCommandHandler : IRequestHandler<UpdateEmailTemplateCommand>
+{
+    private readonly IRepository<EmailTemplate> _repository;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="repository"></param>
+    public UpdateEmailTemplateCommandHandler(IRepository<EmailTemplate> repository)
+    {
+        _repository = repository;
+    }
+
+    public async Task Handle(UpdateEmailTemplateCommand request, CancellationToken cancel)
+    {
+        var temp = (request.EmailTemplateQuery?.Id is int id
+            ? await _repository.ReadOrDefaultAsync<EmailTemplateDto>(t => t.Id == id, single: false, cancel)
+            : request!.EmailTemplateQuery!.Type is Common.Constants.EmailTemplateType type
+                ? await _repository.ReadOrDefaultAsync<EmailTemplateDto>(t => t.Name == type.ToString(), single: false, cancel)
+                : throw new InvalidOperationException("Both id and type is null. Id: " + request.EmailTemplateQuery.Id + ". Type: " + request.EmailTemplateQuery.Type.ToString())) ?? throw new NotFoundException();
+
+        if(request.Body is not null)
+            temp.Body = request.Body;
+
+        if (request.Subject is not null)
+            temp.Subject = request.Subject;
+
+        await _repository.UpdateAsync(temp, t => t.Id == temp.Id, cancel);
+    }
+}