POST /api/oauth/codex/import accepted a payload with an already-invalidated
refresh_token and persisted it as an 'active' connection that could never
work — the failure surfaced confusingly only on first real use, long after
the import looked successful.
Validate each record's refresh_token against OpenAI's OAuth endpoint before
persisting, reusing refreshCodexToken() (free exchange, no quota). On an
unrecoverable refresh error the record is rejected with a clear re-auth
message; a valid token imports as before, with any rotated tokens applied.
Bulk import still processes each record independently — one dead token no
longer blocks the valid ones.
Reproduced live 2026-07-16: a 2026-07-10 auth.json imported clean but its
refresh_token returned 401 refresh_token_invalidated. TDD: 3 tests RED against
the old route, 5/5 GREEN with the fix.
Closes#7522
