Function s3_put_to 

pub(crate) fn s3_put_to(
    agent: &Agent,
    base_url: &str,
    bucket: &str,
    key: &str,
    region: &str,
    body: &[u8],
    creds: &UploadCredentials,
) -> Result<String, String>

Internal: same as s3_put but accepts an explicit base_url. Used in unit tests to point at a plain-HTTP mock server.