Since #35491, register_model records every registration in the process-global
_runtime_registered_model_cost ledger, and every cost map swap replays that
ledger on top of the freshly adopted map. Under pytest-xdist, any earlier test
in the same worker that registered gpt-3.5-turbo leaked into
TestPriceDataReloadIntegration::test_distributed_reload_check_function: the
replay ballooned its sparse mocked entry into a full ModelInfo dict and failed
the exact-equality assert, breaking the proxy-infra shard whenever loadscope
happened to co-schedule such a test first (reruns cannot help since the
pollution is process-wide)
The autouse isolate_litellm_state fixture now snapshots the ledger before each
test and restores it in place on teardown, so no test's registrations outlive
it. A regression pair in test_conftest_isolation.py asserts the rollback
