Refactor EnvelopeStatus usage and streamline code

- Added `EnvelopeGenerator.Domain.Constants` in multiple files, replacing `Domain.Constants.EnvelopeStatus` with `EnvelopeStatus` for improved readability.
- Modified `CreateEnvelopeCommandHandler.cs` to directly use `request.UserId`, eliminating unnecessary local variable assignment.
- Identified potential duplication in query logic within `ReceiverAlreadySignedQuery.cs`.
- Updated `IEnvelopeService`, `EnvelopeReceiverService`, and `EnvelopeService` to ensure consistent usage of the simplified `EnvelopeStatus`.
- Overall, these changes enhance code maintainability and clarity.

EnvelopeGenerator.Application/Envelopes/Commands/CreateEnvelopeCommandHandler.cs

@@ -33,9 +33,7 @@ public class CreateEnvelopeCommandHandler : IRequestHandler<CreateEnvelopeComman
     /// <returns></returns>
     public async Task<EnvelopeDto?> Handle(CreateEnvelopeCommand request, CancellationToken cancellationToken)
     {
-        int userId = request.UserId ?? throw new InvalidOperationException("UserId cannot be null when creating an envelope.");
-
-        var envelope = await _envelopeExecutor.CreateEnvelopeAsync(userId, request.Title, request.Message, request.TFAEnabled, cancellationToken);
+        var envelope = await _envelopeExecutor.CreateEnvelopeAsync(request.UserId, request.Title, request.Message, request.TFAEnabled, cancellationToken);
 
         return _mapper.Map<EnvelopeDto>(envelope);
     }