Function activate_remote_at 

pub(crate) async fn activate_remote_at(
    client: &Client,
    base: &str,
    key: &str,
    instance_name: &str,
) -> Result<String, String>

HTTP-layer split for activate_remote: takes an explicit base URL so tests can point it at mockito::Server without bringing up Lemon Squeezy. The production caller hard-codes LS_API_BASE.