jextract: Preserve underlying exception when async closure completes exceptionally

When a Java async closure completes exceptionally (e.g. with RuntimeException), the error is currently caught in the Swift thunk and completed exceptionally on the Java future as a generic Exception:

Future<Void> future = MySwiftLibrary.asyncScheduleThrowing(() -> {
    CompletableFuture<Void> failed = new CompletableFuture<>();
    failed.completeExceptionally(new RuntimeException("Java async failed"));
    return failed;
});

assertThrows(ExecutionException.class, future::get);

Optimally, we should unwrap the ExecutionException in the Swift thunk and rethrow/preserve the underlying RuntimeException as the cause rather than wrapping it in a generic Exception.

Start in the Swift thunk that catches exceptions from a Java async closure and trace how exceptional completion is propagated to the Java future. Reproduce the RuntimeException case from the issue and verify that the underlying exception is preserved as the cause rather than replaced by a generic Exception.