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.Infrastructure/Executor/EnvelopeReceiverExecutor.cs

@@ -19,7 +19,7 @@ public class EnvelopeReceiverExecutor: SQLExecutor, IEnvelopeReceiverExecutor
         _erRepository = erRepository;
     }
 
-    public async Task<EnvelopeReceiver?> AddEnvelopeReceiverAsync(string envelope_uuid, string emailAddress, string salutation, string? phone = null, CancellationToken cancellation = default)
+    public async Task<EnvelopeReceiver?> AddEnvelopeReceiverAsync(string envelope_uuid, string emailAddress, string? salutation, string? phone = null, CancellationToken cancellation = default)
     {
         using var connection = new SqlConnection(Params.ConnectionString);
         var sql = Provider.GetRequiredService<EnvelopeReceiverAddReadSQL>();