Applies Opus ship-gate findings: (1) the autouse fixture now points
_get_models_cache_path at an isolated temp file so the over-budget static-fallback
test can't read a real ~/.../models_cache.json and become an order-dependent flake;
(2) _load_stale_models_cache_from_disk rejects a cross-schema cache (its groups/badge
shape may be incompatible with the current picker). Adds a cross-schema-rejection test.
Co-authored-by: starship-s <starship-s@users.noreply.github.com>