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/ReadEmailTemplateMappingProfile.cs

@@ -0,0 +1,23 @@
+using AutoMapper;
+using EnvelopeGenerator.Domain.Entities;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace EnvelopeGenerator.Application.EmailTemplates.Queries.Read;
+
+/// <summary>
+/// 
+/// </summary>
+public class ReadEmailTemplateMappingProfile : Profile
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public ReadEmailTemplateMappingProfile()
+    {
+        CreateMap<EmailTemplate, ReadEmailTemplateResponse>();
+    }
+}

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

@@ -1,10 +1,12 @@
-namespace EnvelopeGenerator.Application.EmailTemplates.Queries.Read;
+using MediatR;
+
+namespace EnvelopeGenerator.Application.EmailTemplates.Queries.Read;
 
 
 /// <summary>
 /// Stellt eine Abfrage dar, um eine E-Mail-Vorlage zu lesen.
 /// Diese Klasse erbt von <see cref="EmailTemplateQuery"/>.
 /// </summary>
-public record ReadEmailTemplateQuery : EmailTemplateQuery
+public record ReadEmailTemplateQuery : EmailTemplateQuery, IRequest<ReadEmailTemplateResponse?>
 {
 }

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);
+    }
+}

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

@@ -3,18 +3,35 @@
 /// <summary>
 /// Stellt die Antwort für eine Abfrage von E-Mail-Vorlagen bereit.
 /// </summary>
-/// <param name="Id">Die eindeutige Kennung der E-Mail-Vorlage.</param>
-/// <param name="Type">Der Typ der E-Mail-Vorlage.</param>
-/// <param name="AddedWhen">Das Datum und die Uhrzeit, wann die Vorlage hinzugefügt wurde.</param>
-/// <param name="Body">Der Inhalt (Body) der E-Mail-Vorlage. Kann null sein.</param>
-/// <param name="Subject">Der Betreff der E-Mail-Vorlage. Kann null sein.</param>
-/// <param name="ChangedWhen">Das Datum und die Uhrzeit, wann die Vorlage zuletzt geändert wurde. Kann null sein.</param>
-public record ReadEmailTemplateResponse(
-    int Id,
-    int Type,
-    DateTime AddedWhen,
-    string? Body = null,
-    string? Subject = null,
-    DateTime? ChangedWhen = null)
+public class ReadEmailTemplateResponse
 {
+    /// <summary>
+    /// Die eindeutige Kennung der E-Mail-Vorlage.
+    /// </summary>
+    public int Id { get; init; }
+
+    /// <summary>
+    /// Der Typ der E-Mail-Vorlage.
+    /// </summary>
+    public int Type { get; init; }
+
+    /// <summary>
+    /// Das Datum und die Uhrzeit, wann die Vorlage hinzugefügt wurde.
+    /// </summary>
+    public DateTime AddedWhen { get; init; }
+
+    /// <summary>
+    /// Der Inhalt (Body) der E-Mail-Vorlage. Kann null sein.
+    /// </summary>
+    public string? Body { get; set; }
+
+    /// <summary>
+    /// Der Betreff der E-Mail-Vorlage. Kann null sein.
+    /// </summary>
+    public string? Subject { get; set; }
+
+    /// <summary>
+    /// Das Datum und die Uhrzeit, wann die Vorlage zuletzt geändert wurde. Kann null sein.
+    /// </summary>
+    public DateTime? ChangedWhen { get; set; }
 }