- Introduced QuestPDF dependency to generate PDF documents in tests.
- Added `CreatePdfAsBase64` extension method for generating random PDF content.
- Added `CreateDocumentCommand` and `CreateDocumentCommands` helpers for creating document test data.
- Refactored using statements and added `EnvelopeGenerator.Application.EnvelopeReceivers.Commands`.
- Maintains existing sample user and receiver setup for integration testing.