tests: stabilize api-baseline harness under freezegun + heavy-module stubs
The baseline suite's module-scoped freeze_time ran before the autouse
heavy-module stub fixture, causing freezegun to traverse real
pandas/sklearn/torch and crash on pristine main. Extract the stub install
into a helper, invoke it at module scope for this suite, and pass the
heavy libs as freezegun ignore prefixes.
Reproduces and fixes a test failure that predates the chat-refactor
follow-ups lode.
Co-Authored-By: OpenAI Codex <codex@openai.com>