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

[rishabhjainps]

Motivation and Context

Fixes #7174.

When a multipart download (S3AsyncClient with multipartEnabled(true), e.g. via S3TransferManager.downloadFile) fails, the caller's completion future can complete with a bare java.util.concurrent.CancellationException that has no cause attached, and the real trigger is never logged.

The cause is the ordering in ParallelMultipartDownloaderSubscriber.onError:

public void onError(Throwable t) {
    inFlightRequests.values().forEach(future -> future.cancel(true)); // cancel first
    inFlightRequests.clear();
    resultFuture.completeExceptionally(t);                            // real cause last
}

Each part future is wired to resultFuture with CompletableFutureUtils.forwardExceptionTo(resultFuture, response) in sendNextRequest/sendFirstRequest. Cancelling those part futures therefore forwards a CancellationException onto resultFuture before resultFuture.completeExceptionally(t) runs, so the original t loses the race and is discarded. The method also does not log t, so the trigger is invisible even at DEBUG.

The sibling class ParallelPresignedUrlMultipartDownloaderSubscriber.onError already does this correctly — it completes resultFuture before cancelling and logs the cause. This change brings ParallelMultipartDownloaderSubscriber in line with it.

Modifications

In ParallelMultipartDownloaderSubscriber.onError:

• Complete resultFuture with the original Throwable before cancelling the in-flight part futures, so the real cause wins the race instead of a CancellationException.
• Log the cause at debug level, matching the sibling class.

Testing

• Added ParallelMultipartDownloaderSubscriberTest, which verifies that after onError the caller-facing resultFuture completes with the original throwable (not a CancellationException), that this holds both with and without in-flight part requests, and that in-flight part futures are still cancelled. The test fails against the previous cancel-before-complete ordering and passes with this change.
• Built and tested locally on the services/s3 module with Corretto 21: ./mvnw -pl services/s3 -am install succeeds; -Dtest=ParallelMultipartDownloaderSubscriberTest runs 3/3 green; checkstyle:check on the module is clean.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
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• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license

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