Add method to fetch document bytes by envelope ID

Added `GetSenderDocumentBytesByEnvelopeIdAsync` to the
`DocumentService` class. This method retrieves sender
document bytes using an HTTP GET request to the endpoint
`/api/Document?EnvelopeId={envelopeId}`. It handles HTTP
response errors by throwing an `HttpRequestException` and
supports cancellation via a `CancellationToken`.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/DocumentService.cs

@@ -55,4 +55,26 @@ public class DocumentService(IHttpClientFactory httpClientFactory)
 
         return await response.Content.ReadFromJsonAsync<DocumentDto>(_jsonOptions, cancel);
     }
+
+    /// <summary>
+    /// Fetches sender document bytes by envelope id.
+    /// Uses GET /api/Document?EnvelopeId={envelopeId}.
+    /// </summary>
+    public async Task<byte[]?> GetSenderDocumentBytesByEnvelopeIdAsync(int envelopeId, CancellationToken cancel = default)
+    {
+        using var http = httpClientFactory.CreateClient("EnvelopeGenerator.Server");
+        var response = await http.GetAsync($"/api/Document?EnvelopeId={envelopeId}", cancel);
+
+        if (!response.IsSuccessStatusCode)
+        {
+            var statusCode = (int)response.StatusCode;
+            var reasonPhrase = response.ReasonPhrase ?? "Unknown error";
+            throw new HttpRequestException(
+                $"Failed to load sender document bytes. Status: {statusCode} ({reasonPhrase})",
+                null,
+                response.StatusCode);
+        }
+
+        return await response.Content.ReadAsByteArrayAsync(cancel);
+    }
 }