Function require_createdb 

pub async fn require_createdb(
    client: &Client,
    current_role: &str,
    sample_overlay_db: &str,
) -> Result<(), CliError>

Verify the current role has CREATEDB privilege. mz-deploy dev calls this before attempting to create overlay databases.

sample_overlay_db is used only for the error message, so the user sees a concrete name they’d fail to create.