Fixes the CI failure introduced by #817: test_model_resolver::test_custom_endpoint_uses_model_config_api_key_for_model_discovery was failing with KeyError: 'auth' due to the 60s TTL cache in get_available_models() being populated by test_byok_model_dropdown.py tests that ran earlier. Added autouse _isolate_models_cache fixture to 5 test files. Full suite 1747/1747, QA harness green.
MiniMax M2.7/highspeed added to _FALLBACK_MODELS. MINIMAX_API_KEY and MINIMAX_CN_API_KEY added to env scan tuple so os.environ is checked. 11 tests. Independent review by @nesquena confirmed correct, needed rebase only.
