Added a new HTTP GET action to DocResultController that accepts a ReadSingleEnvelopeDocResultQuery via query string. The endpoint uses MediatR to retrieve the PDF document and returns it as a file response with the envelope's UUID in the filename and the correct content type.
Introduced DocResultController with a GET endpoint to retrieve envelope PDF documents by sending a query via MediatR. The controller returns the PDF as a file response with an appropriate filename and content type. Added necessary using directives for MediatR, ASP.NET Core MVC, and the application query.
