refator(SaveDocStatusCommandHandler): use SingleOrDefaultAsync instead of FirstOrDefaultAsync

EnvelopeGenerator.Application/DocStatus/Commands/SaveDocStatusCommand.cs

@@ -91,7 +91,7 @@ public class SaveDocStatusCommandHandler : IRequestHandler<SaveDocStatusCommand,
             await _repo.CreateAsync(cReq, cancel);
         }
 
-        var docStatus = await _repo.ReadOnly().Where(eExp).Where(rExp).FirstOrDefaultAsync(cancel);
+        var docStatus = await _repo.ReadOnly().Where(eExp).Where(rExp).SingleOrDefaultAsync(cancel);
         return docStatus?.Id;
     }
 }