Single-version 3.12 job hides regressions on floor (3.10) and newly
supported ceiling (3.14). Run all five versions with fail-fast off
so partial breakage is visible.
Depot Startup plan (unlimited minutes) for consistent runner quality
across unit-test workflows. Extends the pattern from PR #4018.
Also adds the id-token: write permission required for Depot OIDC
auth, alongside contents: read for least-privilege defaults.