- 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`.
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