test(gooddata-eval): annotate the conversation tests, drop the openai patches

Type annotations on the helpers and tests added for the context mode,
as AGENTS.md asks of every function. The simulated-user tests patched
`openai.OpenAI`, which fails before the code under test runs wherever
the optional openai package is absent (a root `uv run pytest`); they now
put a stand-in openai module in sys.modules, so they pass with and
without the llm-judge extra.

jira: QA-29448
risk: nonprod

Author: myhoai