- Changed `BurnPdfCommand` from empty class to a `record` with properties: `SourceBuffer`, `InstantJSONList`, `EnvelopeId`.
- Updated `BurnPdfCommandHandler` to implement `IRequestHandler<BurnPdfCommand, byte[]>`.
- Modified `Handle` method to be async and use the `request` data to return PDF bytes.
- Removed obsolete `BurnPdfCommand` class and updated related logic accordingly.