fix: diagnose and fix the 26.09 maas smoke failure

[dholt]

Diagnose and fix the 26.09 MAAS smoke failure

What this is

MAAS controller role on Ubuntu 24.04 (Noble): the add MAAS repository task applied the legacy default ppa:maas/3.5, but MAAS 3.5 publishes no Noble packages, so apt_repository failed on a missing Release file and the MAAS install never started. Normalize that legacy override to ppa:maas/3.7 when the host runs Ubuntu 24.04 or newer, while preserving any custom maas_repo value. QA-fix scope only: touch only files needed for this failure.

Verification

• Deterministic checks passed: diff check, public sanitizer, parent commit
• Two independent agent reviews approved head e98829e1eae8 (different model vendors; strict verdict contract)
• Published by the DeepOps maintenance loop's deterministic publisher after its publication gate (reviews, provenance, exact-head) passed

[dholt]

Closing without merge. Root cause was in our private release-QA harness, which pinned maas_repo: ppa:maas/3.5 into the smoke's extra-vars and overrode the role default. master already selects ppa:maas/3.7 on Ubuntu 24.04 (role default, config.example, and docs agree), so no public change is needed; the harness now lets the role default decide and verifies the repository can serve the host's Ubuntu series before installing.