Function create_folder 

pub async fn create_folder(
    client: &Client,
    bucket: &str,
    prefix: &str,
) -> Result<(), AppError>

Create a virtual folder placeholder at bucket/prefix/.

Issues a zero-byte PutObject with key = "{prefix}/". If the object already exists S3 overwrites it silently — the operation is idempotent.

Errors

• AppError::AccessDenied — s3:PutObject permission denied.
• AppError::NotFound — bucket does not exist.
• AppError::Network — any other SDK or transport error.