Refactor envelope doc query and improve result validation

Refactored ReadSingleEnvelopeDocResultQuery to remove inheritance from EnvelopeQueryBase and introduce an Envelope property. Enhanced the handler to ensure DocResult is a non-empty byte array before returning, throwing NotFoundException otherwise.

EnvelopeGenerator.Application/Envelopes/Queries/ReadSingleEnvelopeDocResultQuery.cs

@@ -1,5 +1,4 @@
 using MediatR;
-using EnvelopeGenerator.Application.Common.Query;
 using EnvelopeGenerator.Application.Common.Dto;
 using DigitalData.Core.Exceptions;
 
@@ -8,7 +7,7 @@ namespace EnvelopeGenerator.Application.Envelopes.Queries;
 /// <summary>
 /// Repräsentiert eine Abfrage für Umschläge.
 /// </summary>
-public record ReadSingleEnvelopeDocResultQuery() : EnvelopeQueryBase, IRequest<byte[]>
+public record ReadSingleEnvelopeDocResultQuery() : IRequest<byte[]>
 {
     /// <summary>
     /// 
@@ -41,6 +40,8 @@ public class ReadSingleEnvelopeDocResultQueryHandler : IRequestHandler<ReadSingl
     public async Task<byte[]> Handle(ReadSingleEnvelopeDocResultQuery request, CancellationToken cancellationToken)
     {
         var result = await _mediator.Send(request.Envelope, cancellationToken);
-        return result.DocResult ?? throw new NotFoundException($"Document for Envelope with ID {request.Envelope.Id} not found");
+        return result.DocResult is byte[] docResult && docResult.Length > 0 
+            ? docResult 
+            : throw new NotFoundException($"Document for Envelope with ID {request.Envelope.Id} not found");
     }
 }