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.
Providers in config.yaml with explicit models: list were silently ignored. Fix extends the model-list builder to check cfg.providers[pid].models, covering both dict and list formats. Also includes providers only in config.yaml (not _PROVIDER_MODELS). 5 regression tests added. Independent review by @nesquena.
