feat(SendEmailBehavior): add SendEmailBehavior for sending final emails after PDF burn

- Implements IPipelineBehavior<BurnPdfCommand, byte[]> to send emails to creator and receivers.
- Logs debug information when sending emails and warnings if email sending is skipped.
- Uses Envelope's FinalEmailType to determine whether to send emails.

EnvelopeGenerator.Application/DependencyInjection.cs

@@ -91,6 +91,7 @@ public static class DependencyInjection
             cfg.AddBehavior<CreateHistoryBehavior>();
             cfg.AddBehavior<SavePdfBehavior>();
 #if WINDOWS
+            cfg.AddBehavior<SendEmailBehavior>();
             cfg.AddBehavior<WritePdfBehavior>();
             cfg.AddBehavior<PdfMergeBehavior>();
             cfg.AddBehavior<AddReportBehavior>();