- Changed return type of `CreateEnvelopeAsync` to ensure a non-null `Envelope` is always returned.
- Updated `AddEnvelopeReceiverAsync` to allow nullable `salutation` parameter.
- Renamed cancellation token parameter in `CreateEnvelopeReceiverCommandHandler` for consistency.
- Enhanced error handling in `CreateEnvelopeAsync` to throw exceptions on failure with improved messages.
- Adjusted `CreateParameters` method to accept nullable `salutation`.
This commit introduces two new dependencies: `IEnvelopeExecutor` and `IEnvelopeReceiverExecutor`. The `using` directive for `IEnvelopeExecutor` has been added, and a new private field `_erExecutor` has been introduced in the `CreateEnvelopeReceiverCommandHandler` class. A constructor has also been added to initialize both `_envelopeExecutor` and `_erExecutor`, enabling the handler to effectively process envelope-related commands.
