Simplify payload handling in UpsertAsync method

Refactored the `UpsertAsync` method in `EnvelopeService` to remove
manual construction of the HTTP POST payload. The method now
directly uses the `CreateEnvelopeCommand` object as the payload
for the `PostAsJsonAsync` call. This change reduces redundancy,
simplifies the code, and leverages the existing object structure
for better maintainability.

EnvelopeGenerator.Server/EnvelopeGenerator.Server.Client/Services/EnvelopeService.cs

@@ -101,26 +101,7 @@ public class EnvelopeService(IHttpClientFactory clientFactory)
         CancellationToken cancel = default)
     {
         var httpClient = clientFactory.CreateClient("EnvelopeGenerator.Server");
-        var payload = new
+        var response = await httpClient.PostAsJsonAsync("/api/Envelope", request, _jsonOptions, cancel);
-        {
-            EnvelopeId = request.EnvelopeId,
-            Title = request.Title,
-            Message = request.Message,
-            TFAEnabled = request.TFAEnabled,
-            Send = request.Send,
-            Document = request.Document is null
-                ? null
-                : new { DataAsBase64 = request.Document.DataAsBase64 },
-            Receivers = request.Receivers.Select(r => new
-            {
-                EmailAddress = r.EmailAddress,
-                Name = r.Name,
-                PhoneNumber = r.PhoneNumber,
-                Fields = r.Fields.Select(f => new { X = f.X, Y = f.Y, Page = f.Page }).ToList()
-            }).ToList()
-        };
-
-        var response = await httpClient.PostAsJsonAsync("/api/Envelope", payload, _jsonOptions, cancel);
 
         if (response.IsSuccessStatusCode)
             return await response.Content.ReadFromJsonAsync<EnvelopeDto>(_jsonOptions, cancel);