Salvage of the Gemini-specific piece from PR #12585 by @briandevans.
Gemini's OpenAI-compat /v1beta/openai/models endpoint returns IDs prefixed
with 'models/' (native Gemini-API convention), so set-membership against
curated bare IDs drops every model. Strip the prefix before comparison.
The Anthropic static-catalog piece of #12585 was subsumed by #12618's
_fetch_anthropic_models() branch landing earlier in the same salvage PR.
Full branch cherry-pick was skipped because it also carried unrelated
catalog-version regressions.