This commit modifies the audio download process in `whatsapp-router/src/webhook.ts` to align with the updated nucleo-whatsapp API.
Previously, audio files were downloaded using a direct GET request to the file URL. This has been changed to a POST request to the `/downloadFileWithCredentials` endpoint provided by nucleo-whatsapp.
Key changes:
- Audio files are now downloaded by POSTing to `[OPEN_WA_URL]/downloadFileWithCredentials` with the audio file's URL in the request body.
- The `/downloadFileWithCredentials` endpoint returns a base64 encoded string directly, so the explicit base64 conversion step after downloading has been removed.
This change ensures compatibility with the correct API for fetching message media.
