- detect_latest_debian_template now selects the newest debian-12 template only.
Auto-detecting the newest Debian overall picked debian-13, which would pair the
deb12-only DocumentDB extension with a mismatched libicu. Trixie now requires
setting TEMPLATE + DOCUMENTDB_DISTRO explicitly.
- Add preflight_script_url: verify SCRIPT_URL is reachable on the host before
creating the LXC, so a branch/path that 404s fails immediately with guidance
instead of dying mid-install after the container is built.