Add MediatR query handler for reading receiver details

Introduced a CQRS-style ReadReceiverQueryHandler using MediatR to enable flexible querying of receivers by Id, EmailAddress, or Signature. Integrated AutoMapper for DTO mapping and updated ReadReceiverQuery to support MediatR's request/response pipeline.

EnvelopeGenerator.Application/Receivers/Queries/ReadReceiverQuery.cs

@@ -1,4 +1,10 @@
-using EnvelopeGenerator.Application.Common.Query;
+using AutoMapper;
+using DigitalData.Core.Abstraction.Application.Repository;
+using EnvelopeGenerator.Application.Common.Dto.Receiver;
+using EnvelopeGenerator.Application.Common.Query;
+using MediatR;
+using EnvelopeGenerator.Domain.Entities;
+using Microsoft.EntityFrameworkCore;
 
 namespace EnvelopeGenerator.Application.Receivers.Queries;
 
@@ -6,4 +12,53 @@ namespace EnvelopeGenerator.Application.Receivers.Queries;
 /// Stellt eine Abfrage dar, um die Details eines Empfängers zu lesen.
 /// um spezifische Informationen über einen Empfänger abzurufen.
 /// </summary>
-public record ReadReceiverQuery : ReceiverQueryBase;
+public record ReadReceiverQuery : ReceiverQueryBase, IRequest<IEnumerable<ReceiverDto>>;
+
+/// <summary>
+/// 
+/// </summary>
+public class ReadReceiverQueryHandler : IRequestHandler<ReadReceiverQuery, IEnumerable<ReceiverDto>>
+{
+    private readonly IRepository<Receiver> _repository;
+    private readonly IMapper _mapper;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="repository"></param>
+    /// <param name="mapper"></param>
+    public ReadReceiverQueryHandler(IRepository<Receiver> repository, IMapper mapper)
+    {
+        _repository = repository;
+        _mapper = mapper;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public async Task<IEnumerable<ReceiverDto>> Handle(ReadReceiverQuery request, CancellationToken cancellationToken)
+    {
+        var query = _repository.Query;
+
+        if (request.Id is int id)
+        {
+            query = query.Where(r => r.Id == id);
+        }
+
+        if (request.EmailAddress is string email)
+        {
+            query = query.Where(r => r.EmailAddress == email);
+        }
+
+        if (request.Signature is string signature)
+        {
+            query = query.Where(r => r.Signature == signature);
+        }
+
+        var receiver = await query.ToListAsync(cancellationToken);
+        return _mapper.Map<IEnumerable<ReceiverDto>>(receiver);
+    }
+}