feat: add GetDocumentOfSenderAsync to EnvelopeReceiverPageDataService

Loads the PDF bytes for a given envelope on behalf of the authenticated
sender (uses SenderId() claim extension). Throws NotFoundException when
no document data is available so callers can surface a typed error.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Services/EnvelopeReceiverPageDataService.cs

@@ -1,5 +1,6 @@
 using System.Security.Claims;
 using System.Text.Json;
+using DigitalData.Core.Exceptions;
 using EnvelopeGenerator.Application.Common.Dto;
 using EnvelopeGenerator.Application.Common.Dto.EnvelopeReceiver;
 using EnvelopeGenerator.Application.Documents.Queries;
@@ -37,6 +38,17 @@ public class EnvelopeReceiverPageDataService(
         return document.ByteData;
     }
 
+    /// <summary>
+    /// Loads the PDF document bytes for the authenticated receiver.
+    /// </summary>
+    public async Task<byte[]?> GetDocumentOfSenderAsync(ClaimsPrincipal user, int envelopeId, CancellationToken cancellationToken = default)
+    {
+        var document = await mediator.Send(new ReadDocumentQuery(EnvelopeId: envelopeId, UserId: user.SenderId()), cancellationToken);
+        return (document.ByteData is byte[] data && data.Length > 0)
+            ? data
+            : throw new NotFoundException($"Document for envelope {envelopeId} not found.");
+    }
+
     /// <summary>
     /// Loads the current receiver's signature placeholders.
     /// </summary>