fix(csharp): always complete result queue when download loop exits

The download loop's finally block only called CompleteAdding() on the
result queue when the downloader itself had an error. When the fetcher
failed mid-stream (e.g. warehouse stopped), the downloader had no error
of its own, so the queue was left open — empty but not completed.

On retry, BlockingCollection.Take() blocked forever waiting for items
that would never arrive, deadlocking the Mashup Container thread and
causing Power BI to hang with a spinner instead of showing an error.

Now CompleteAdding() is called on every exit path, so Take() on an
empty queue throws InvalidOperationException (caught by existing
handler) instead of blocking, allowing the fetcher error to propagate.

Co-authored-by: Isaac

Author: eric-wang-1990

csharp/src/Reader/CloudFetch/CloudFetchDownloader.cs

@@ -437,11 +437,20 @@ await _activityTracer.TraceActivityAsync(async activity =>
                         new("total_time_sec", overallStopwatch.ElapsedMilliseconds / 1000.0)
                     ]);
 
-                    // If there's an error, add the error to the result queue
+                    // Always mark the result queue as complete when the download
+                    // loop exits. Without this, a subsequent Take() call would
+                    // block forever on an empty, non-completed queue if the caller
+                    // retries after an exception (e.g. a fetcher error that the
+                    // downloader doesn't know about).
                     if (HasError)
                     {
                         CompleteWithError(activity);
                     }
+                    else
+                    {
+                        try { _resultQueue.CompleteAdding(); }
+                        catch (InvalidOperationException) { /* already completed */ }
+                    }
                 }
             });
         }