Refactor envelope and receiver methods for clarity

- 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`.

EnvelopeGenerator.Application/Contracts/SQLExecutor/IEnvelopeReceiverExecutor.cs

@@ -16,5 +16,5 @@ public interface IEnvelopeReceiverExecutor
     /// <param name="phone"></param>
     /// <param name="cancellation"></param>
     /// <returns></returns>
-    Task<EnvelopeReceiver?> AddEnvelopeReceiverAsync(string envelope_uuid, string emailAddress, string salutation, string? phone = null, CancellationToken cancellation = default);
+    Task<EnvelopeReceiver?> AddEnvelopeReceiverAsync(string envelope_uuid, string emailAddress, string? salutation = null, string? phone = null, CancellationToken cancellation = default);
 }