feat(WritePdfBehavior): add WritePdfBehavior to handle PDF writing pipeline

- Implements WritePdfBehavior as an IPipelineBehavior for BurnPdfCommand
- Writes generated PDF to configured export path
- Automatically creates export directory if missing
- Adds logging for export path and file operations

EnvelopeGenerator.Application/DependencyInjection.cs

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