Add DocResultController for envelope PDF download

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.

EnvelopeGenerator.ServiceHost/Controllers/DocResultController.cs

@@ -0,0 +1,15 @@
+using EnvelopeGenerator.Application.Envelopes.Queries;
+using MediatR;
+using Microsoft.AspNetCore.Mvc;
+
+namespace EnvelopeGenerator.ServiceHost.Controllers;
+
+[Route("api/[controller]")]
+[ApiController]
+public class DocResultController(IMediator mediator) : ControllerBase
+{
+    public async Task<IActionResult> GetAsync([FromQuery] ReadSingleEnvelopeDocResultQuery query, CancellationToken cancel = default)
+    {
+        return File(await mediator.Send(query, cancel), "application/pdf", $"envelope_{query.Envelope.Uuid}.pdf");
+    }
+}