docs: fix stale ModelAPI docstrings (nonexistent api_key_vars / ChatUserMessage)

[BingH225]

Description

Two stale docstrings in the ModelAPI base class describe an API that does not exist:

1. ModelAPI.__init__ documents an api_key_vars parameter that is not in the
   signature. grep -rn "api_key_vars" matches only this docstring — the
   parameter does not exist anywhere in the codebase.
2. ModelAPI.generate documents input as str | list[ChatMessage] and claims a
   str is converted to a ChatUserMessage. But:
   • the signature is input: List[ChatMessage] and does not accept str;
   • str -> message conversion happens one layer up in Model._preprocess_input,
     not in this base class;
   • ChatUserMessage does not exist; the actual class is ChatMessageUser
     (433 occurrences), so the name in the docstring is wrong as well.

Both appear to be leftovers from the upstream inspect-ai API.

Test

Docstring only, no behavior change.

• make lint equivalents pass: ruff check evalscope/api/model/model.py reports no new
  findings vs. the unmodified baseline; yapf --diff -> empty
• python -m py_compile passes
• No auto-generated files touched (docs/{zh,en}/benchmarks/, evalscope/benchmarks/_meta/)

[Yunnglin]

LGTM