Refactor return value in EnvelopeReceiverController

Modified the method to return a single item or null by using
`SingleOrDefault()` on the result of `await _mediator.Send(...)`.
This ensures the response is no longer a collection but a single
object, improving clarity and aligning with expected behavior.

EnvelopeGenerator.API/Controllers/EnvelopeReceiverController.cs

@@ -89,7 +89,7 @@ public class EnvelopeReceiverController : ControllerBase
             Key = envelopeKey
         }, cancel);
 
-        return Ok(er);
+        return Ok(er.SingleOrDefault());
     }
 
     /// <summary>