refactor(email): replace Task.WhenAll with foreach in SendEmailBehavior

Changed the SendFinalEmailToReceiversAsync method to use a sequential `foreach` loop
instead of `Task.WhenAll` for sending history commands. Added a null check for
receiver emails to prevent potential exceptions.

EnvelopeGenerator.Application/Pdf/BurnPdfCommand.cs

@@ -110,6 +110,8 @@ public class BurnPdfCommandHandler : IRequestHandler<BurnPdfCommand, byte[]>
 
         request.Envelope = await envQuery
             .Include(env => env.Documents!).ThenInclude(doc => doc.Elements!).ThenInclude(element => element.Annotations)
+            .Include(env => env.User)
+            .Include(env => env.EnvelopeReceivers!).ThenInclude(envRcv => envRcv.Receiver)
             .FirstOrDefaultAsync(cancel)
             ?? throw new BadRequestException($"Envelope could not be found. Request details:\n" +
                 request.ToJson(Format.Json.ForDiagnostics));