Add query/handler for envelope document retrieval

Introduced ReadSingleEnvelopeDocResultQuery and its handler to fetch an envelope's document as a byte array via MediatR. Throws NotFoundException if the document is missing. Includes XML documentation for clarity.

EnvelopeGenerator.Application/Envelopes/Queries/ReadSingleEnvelopeDocResultQuery.cs

@@ -0,0 +1,41 @@
+using MediatR;
+using EnvelopeGenerator.Application.Common.Query;
+using EnvelopeGenerator.Application.Common.Dto;
+using DigitalData.Core.Exceptions;
+
+namespace EnvelopeGenerator.Application.Envelopes.Queries;
+
+/// <summary>
+/// Repräsentiert eine Abfrage für Umschläge.
+/// </summary>
+/// <param name="Envelope"></param>
+public record ReadSingleEnvelopeDocResultQuery(ReadSingleEnvelopeQuery Envelope) : EnvelopeQueryBase, IRequest<byte[]>;
+
+/// <summary>
+/// Verarbeitet <see cref="ReadEnvelopeQuery"/> und liefert passende <see cref="EnvelopeDto"/>-Ergebnisse.
+/// </summary>
+public class ReadSingleEnvelopeDocResultQueryHandler : IRequestHandler<ReadSingleEnvelopeDocResultQuery, byte[]>
+{
+    private readonly IMediator _mediator;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="mediator"></param>
+    public ReadSingleEnvelopeDocResultQueryHandler(IMediator mediator)
+    {
+        _mediator = mediator;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public async Task<byte[]> Handle(ReadSingleEnvelopeDocResultQuery request, CancellationToken cancellationToken)
+    {
+        var result = await _mediator.Send(request.Envelope, cancellationToken);
+        return result.DocResult ?? throw new NotFoundException($"Document for Envelope with ID {request.Envelope.Id} not found");
+    }
+}