fix(api): fail fast on unexpected null document payload

src/presentation/DigitalData.TaskFlow.API/Controllers/DocumentsController.cs

@@ -27,13 +27,7 @@ public sealed class DocumentsController(ISender sender) : ControllerBase
     public async Task<IActionResult> GetAsync([FromRoute] string id, CancellationToken cancellationToken)
     {
         var response = await sender.Send(new GetDocumentQuery(id), cancellationToken);
-        var data = response.Data;
-
-        if (data is null)
-        {
-            return NotFound(response);
-        }
-
+        var data = response.Data ?? throw new InvalidOperationException("Document payload is unexpectedly null.");
         return File(data.Content, data.ContentType, data.FileName);
     }
 }