The two facades post-#102117 (chat_completion_helpers, agent_runtime_helpers) must not grow
behaviour; the marker/predicate belong with the sibling that already owns primary-cooldown
state shared by the fallback walk and restore_primary_runtime. Also drops the two
try/except-return-False wrappers around pool.entries() and normalize_model_for_provider
(neither raises for a live pool / the never-raising normalizer).
Tests trimmed to the two invariants (#106475): the marker fires only on the single-credential
Codex entitlement 400 and the walk skips the rejected slug in either form; restore is gated on
a rejected primary and still restores an unrelated one.
teknium1