Add GET endpoint to return envelope PDF by UUID

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.

EnvelopeGenerator.ServiceHost/Controllers/DocResultController.cs

@@ -8,6 +8,7 @@ namespace EnvelopeGenerator.ServiceHost.Controllers;
 [ApiController]
 public class DocResultController(IMediator mediator) : ControllerBase
 {
+    [HttpGet]
     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");