[Bug] OneDrive ListRootFolder returns 502 BadGateway from API Hub

## Summary

The `onedrive/root` endpoint consistently returns 502 BadGateway from the API Hub, both locally and in Azure deployment.

## Repro

`GET https://sdk-connector-samples.azurewebsites.net/api/onedrive/root`

## Error

The connector calls `GET /datasets/default/folders` which returns 502 from the API Hub gateway (source: `0a8e3a9fa83240839c163637de5e83f0.08.common.logic-df.azure-apihub.net`, innerError status 500).

## Questions to investigate

1. Is `ListRootFolderAsync()` the right API - or should `GetFilesFromFolderAsync()` be used instead?
2. Does `/datasets/default/folders` work with MSI-authenticated connections?
3. Is this a transient API Hub issue or a connection configuration problem?

## Connection details
- Namespace: `sdk-test-gateway` (nsUrlId: `0a8e3a9fa83240839c163637de5e83f0`)
- Connection: `onedriveforbusiness-test`
- MSI ACL: granted (objectId `d77681c2-bb7f-4b90-ba93-2b9cee96cbae`)