The salvaged compatibility test stubs `_is_codex_backend=lambda: False` on a
SimpleNamespace, so it proves the helper honors its own boolean but not that
the boolean is right for any real endpoint. A predicate change that widened
the drop onto retention-supporting hosts would keep it green.
Adds a parametrized test that builds a real AIAgent per base URL and asserts
the drop only fires for chatgpt.com/backend-api/codex, while api.meta.ai,
bedrock-mantle.*.api.aws, api.openai.com and a same-host/different-path
backend keep their supported 24h value. Also asserts prompt_cache_key
survives untouched on every endpoint, since retention and cache-key routing
are independent and the guard must not disturb caching.
Verified non-vacuous: relaxing the guard's condition to drop on every
endpoint fails 4 of the 6 cases (Meta, Bedrock, OpenAI, non-codex path).
Drive-by on the guard itself: drop the dead `None` default on the `pop` that
is already gated by an `in` check, and record why the predicate is resolved
via getattr -- run_codex_stream is driven with lightweight stand-in agents
that lack `_is_codex_backend`, so a bare call would raise AttributeError.
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