Refactor email template query handling with MediatR

- Updated `ReadEmailTemplateQuery` to implement `IRequest<ReadEmailTemplateResponse?>`.
- Changed `ReadEmailTemplateResponse` from a record to a class with updated properties.
- Enhanced `EmailTemplateController` to inject `IEmailTemplateRepository` and `IMediator`, and made the `Get` method asynchronous.
- Introduced `ReadEmailTemplateMappingProfile` for AutoMapper mappings.
- Added `ReadEmailTemplateQueryHandler` to manage query logic and response mapping.

These changes improve the structure and maintainability of the email template querying process.

EnvelopeGenerator.Application/EmailTemplates/Queries/Read/ReadEmailTemplateQueryHandler.cs

@@ -0,0 +1,50 @@
+using AutoMapper;
+using EnvelopeGenerator.Application.Contracts.Repositories;
+using EnvelopeGenerator.Application.DTOs;
+using EnvelopeGenerator.Common;
+using MediatR;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Logging;
+
+namespace EnvelopeGenerator.Application.EmailTemplates.Queries.Read;
+
+/// <summary>
+/// 
+/// </summary>
+public class ReadEmailTemplateQueryHandler : IRequestHandler<ReadEmailTemplateQuery, ReadEmailTemplateResponse?>
+{
+    private readonly IMapper _mapper;
+
+    private readonly IEmailTemplateRepository _repository;
+
+    /// <summary>
+    /// Initialisiert eine neue Instanz der <see cref="EmailTemplateController"/>-Klasse.
+    /// </summary>
+    /// <param name="mapper">
+    /// <param name="repository">
+    /// Die AutoMapper-Instanz, die zum Zuordnen von Objekten verwendet wird.
+    /// </param>
+    public ReadEmailTemplateQueryHandler(IMapper mapper, IEmailTemplateRepository repository)
+    {
+        _mapper = mapper;
+        _repository = repository;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    /// <exception cref="InvalidOperationException"></exception>
+    public async Task<ReadEmailTemplateResponse?> Handle(ReadEmailTemplateQuery request, CancellationToken cancellationToken)
+    {
+        var temp = request.Id is int id
+            ? await _repository.ReadByIdAsync(id)
+            : request.Type is Constants.EmailTemplateType type
+                ? 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);
+    }
+}