test(integration): hold genai client across parity cases (req_bfbdbux6 Lode 1)
The parametrized schema-provider-parity test built a per-call inline
genai.Client(...).models... temporary that was closed before the request
completed ('Cannot send a request, as the client has been closed'), failing
all 13 google cases spuriously. Cache the client per api_key. Test-only;
no product code, schema, or fallback change. Re-verified 39/39 native
(13 Class-A x Google/OpenAI-strict/Anthropic-strict, raw SDK, no fallback).
make ci green.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>