Refactored the `Report` property to `_report` with nullable support
and updated `EnvelopeKey` to use `init` for immutability.
Made `CreateReport` asynchronous, returning `Task<XtraReport>`,
and removed redundant `BasePdfBytes` property. Simplified predefined
report fetching by removing `ReportStorage.TryGetReport`. Improved
error handling for missing or invalid `pdfBytes` in `CreateReport`.
Made minor formatting and structural improvements for clarity.
TekH
88b196ed6d