Add DocumentController for envelope document retrieval

Introduced DocumentController to provide a secured GET endpoint for authenticated receivers to download envelope documents. Handles missing or empty documents with error logging and NotFoundException. Utilizes MediatR and ILogger via dependency injection.

EnvelopeGenerator.GeneratorAPI/Controllers/DocumentController.cs

@@ -0,0 +1,43 @@
+using DigitalData.Core.Exceptions;
+using EnvelopeGenerator.Application.Common.Extensions;
+using EnvelopeGenerator.Application.EnvelopeReceivers.Queries;
+using EnvelopeGenerator.Domain.Constants;
+using MediatR;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+
+namespace EnvelopeGenerator.GeneratorAPI.Controllers;
+
+/// <summary>
+/// Provides access to envelope documents for authenticated receivers.
+/// </summary>
+/// <remarks>
+/// Initializes a new instance of the <see cref="DocumentController"/> class.
+/// </remarks>
+[Authorize(Roles = ReceiverRole.FullyAuth)]
+[ApiController]
+[Route("api/[controller]")]
+public class DocumentController(IMediator mediator, ILogger<DocumentController> logger) : ControllerBase
+{
+    /// <summary>
+    /// Returns the document bytes for the specified envelope receiver key.
+    /// </summary>
+    /// <param name="query">Encoded envelope key.</param>
+    /// <param name="cancel">Cancellation token.</param>
+    [HttpGet]
+    public async Task<IActionResult> GetDocument(ReadEnvelopeReceiverQuery query, CancellationToken cancel)
+    {
+        var envRcv = await mediator.Send(query, cancel).FirstAsync(Exceptions.NotFound);
+
+        var byteData = envRcv.Envelope?.Documents?.FirstOrDefault()?.ByteData;
+
+        if (byteData is null || byteData.Length == 0)
+        {
+            logger.LogError("Document byte data is null or empty for envelope-receiver entity:\n{envelopeKey}.",
+                envRcv.ToJson(Format.Json.ForDiagnostics));
+            throw new NotFoundException("Document is empty.");
+        }
+
+        return File(byteData, "application/octet-stream");
+    }
+}