Resolve the model id for custom/proxy providers by ENDPOINT PROVENANCE
instead of a catalog-family guess. The old gate stripped a vendor prefix
whenever the bare id was first-party of the prefix's home vendor, which
could not distinguish a namespace the proxy actually routes on
(x-ai/grok-4.5 -> keep) from a stale cross-provider leftover on a relay
that only serves bare ids (openai/gpt-5.4 -> strip). A model graduating
into the first-party catalog (agent commit adding grok-4.5) silently
flipped a working custom-proxy id from preserved to stripped -> HTTP 400.
New _endpoint_advertised_model_ids() reads the already-published catalog
snapshot (never builds/probes/touches disk; O(1) per send via a
snapshot-identity memo) and answers what the endpoint advertised:
- full vendor/model advertised -> preserve (#5979, #3872, #548)
- only the bare id advertised -> strip the redundant prefix (#433)
- cold/unbuilt catalog -> preserve verbatim (fail-safe toward
the id the user actively selected)
The non-custom first-party-proxy strip arm is unchanged.
Rewrites the two pinned #433 tests to model provenance (bare-only
advertised) and adds #5979 coverage: advertised-full preserve (bare +
named custom), cold-catalog preserve-verbatim, and both-advertised
prefers-full.
Thanks @b3nw for the report and precise root-cause analysis.
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