Add query for sensitive envelope receiver data

Introduced `ReadEnvelopeReceiverSecretQuery` to fetch sensitive
fields (e.g., access code, phone number) for envelope receivers.
Added extension methods for dispatching the query via `IMediator`
and implemented `ReadEnvelopeReceiverSecretQueryHandler` to
process the query using repositories and AutoMapper.

Updated `EnvelopeController` with a new HTTP GET endpoint
`EnvelopeReceiverWithSecretByMediatr` to expose the query
functionality. This endpoint returns sensitive data as
`EnvelopeReceiverSecretDto` or a 404 response if no match is found.

These changes improve modularity, testability, and separation of
concerns by leveraging MediatR and CQRS patterns.

EnvelopeGenerator.Web/Controllers/EnvelopeController.cs

@@ -438,4 +438,10 @@ public class EnvelopeController : ViewControllerBase
             return this.ViewDocumentNotFound();
         }
     }
+
+    [HttpGet("EnvelopeReceiverWithSecretByMediatr")]
+    public async Task<IActionResult> EnvelopeReceiverWithSecretByMediatr([FromQuery] ReadEnvelopeReceiverSecretQuery q, CancellationToken cancel)
+    {
+        return await _mediator.Send(q, cancel) is EnvelopeReceiverSecretDto dto ? Ok(dto) : NotFound();
+    }
 }