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.
A Codex ChatGPT-account 400 ('The X model is not supported when using Codex
with a ChatGPT account.') names the model, so with a single credential the
slug is dead for that account. The fallback walk still re-selected it and
restore_primary_runtime switched back to the primary at the start of every
turn, announcing an unverified 'Primary model restored' — the two warnings
alternated forever with zero delivered answers (#106475).
Record the rejected (provider, model) pair on the non-retryable client-error
path (only when no multi-credential pool exists — rotation covers that case,
#71970), skip rejected entries during the fallback walk, and gate
restore_primary_runtime on the primary's slug so the session fails closed
with the terminal entitlement error instead of oscillating. Fixes#106475.
(cherry picked from commit 471435b10288f15387b2549a8b02551d82c0f670)
