jextract: Preserve underlying exception when async closure completes exceptionally

[amanmaurya92]

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.