Add query handler to fetch receiver's latest used name

Introduced ReadReceiverNameQueryHandler to retrieve the most recently used name ("Anrede") for a receiver, supporting envelope generation. Updated ReadReceiverNameQuery to use MediatR and CQRS patterns, and implemented repository-based lookups for receiver identification and name retrieval. Added necessary using directives for repository and MediatR support.

EnvelopeGenerator.Application/Envelopes/Queries/ReadReceiverNameQuery.cs

@@ -1,4 +1,8 @@
-using EnvelopeGenerator.Application.Receivers.Queries;
+using DigitalData.Core.Abstraction.Application.Repository;
+using EnvelopeGenerator.Application.Common.Query;
+using MediatR;
+using EnvelopeGenerator.Domain.Entities;
+using Microsoft.EntityFrameworkCore;
 
 namespace EnvelopeGenerator.Application.Envelopes.Queries;
 
@@ -6,6 +10,54 @@ namespace EnvelopeGenerator.Application.Envelopes.Queries;
 /// Eine Abfrage, um die zuletzt verwendete Anrede eines Empfängers zu ermitteln, 
 /// damit diese für zukünftige Umschläge wiederverwendet werden kann.
 /// </summary>
-public record ReadReceiverNameQuery() : ReadReceiverQuery
+public record ReadReceiverNameQuery() : ReceiverQueryBase, IRequest<string?>;
+
+/// <summary>
+/// 
+/// </summary>
+public class ReadReceiverNameQueryHandler : IRequestHandler<ReadReceiverNameQuery, string?>
 {
-}
+    private readonly IRepository<EnvelopeReceiver> _envelopeReceiverRepository;
+    private readonly IRepository<Receiver> _receiverRepository;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="envelopeReceiverRepository"></param>
+    /// <param name="receiverRepository"></param>
+    public ReadReceiverNameQueryHandler(IRepository<EnvelopeReceiver> envelopeReceiverRepository, IRepository<Receiver> receiverRepository)
+    {
+        _envelopeReceiverRepository = envelopeReceiverRepository;
+        _receiverRepository = receiverRepository;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public async Task<string?> Handle(ReadReceiverNameQuery request, CancellationToken cancellationToken)
+    {
+        var receiverQuery = _receiverRepository.Query.AsNoTracking();
+
+        if (request.Id is int id)
+            receiverQuery = receiverQuery.Where(r => r.Id == id);
+        if (request.EmailAddress is string email)
+            receiverQuery = receiverQuery.Where(r => r.EmailAddress == email);
+        if (request.Signature is string signature)
+            receiverQuery = receiverQuery.Where(r => r.Signature == signature);
+
+        var receiver = await receiverQuery.FirstOrDefaultAsync(cancellationToken);
+        if (receiver is null)
+            return null;
+
+        var erName = await _envelopeReceiverRepository.Query
+            .Where(er => er.ReceiverId == receiver.Id)
+            .OrderByDescending(er => er.AddedWhen)
+            .Select(er => er.Name)
+            .FirstOrDefaultAsync(cancellationToken);
+
+        return erName;
+    }
+}