fix(s3): preserve original cause in ParallelMultipartDownloaderSubscriber.onError

onError cancelled in-flight part requests before completing resultFuture with
the failure cause. The cancel loop races a CancellationException onto
resultFuture (via the forwardExceptionTo wiring), so callers observing the
download's completion future saw a bare CancellationException with no root
cause. The trigger was also never logged.

Complete resultFuture with the original throwable first, then cancel, and log
the cause at debug level, matching the sibling
ParallelPresignedUrlMultipartDownloaderSubscriber.

Author: Rishabh Jain

.changes/next-release/bugfix-AmazonS3-4e9e1fe.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "Amazon S3",
+    "contributor": "rishabhjainps",
+    "description": "Fixed an issue in multipart download where a failed download could surface a bare CancellationException with no root cause. ParallelMultipartDownloaderSubscriber.onError now completes the result future with the original throwable before cancelling in-flight part requests, matching the sibling ParallelPresignedUrlMultipartDownloaderSubscriber, and logs the cause at debug level."
+}

services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/ParallelMultipartDownloaderSubscriber.java

@@ -412,12 +412,12 @@ private boolean shouldProcessPendingTransformers() {
 
     @Override
     public void onError(Throwable t) {
-        // Signal received from the publisher this is subscribed to
-        // (in the case of file download, that's FileAsyncResponseTransformerPublisher)
-        // Failed state, something really wrong has happened, cancel everything
+        // Complete the result future with the original cause before cancelling in-flight parts.
+        // Cancelling first races a CancellationException onto resultFuture and masks t.
+        log.debug(() -> "Error in parallel multipart download", t);
+        resultFuture.completeExceptionally(t);
         inFlightRequests.values().forEach(future -> future.cancel(true));
         inFlightRequests.clear();
-        resultFuture.completeExceptionally(t);
     }
 
     @Override