[codex] refactor inspect eval kwargs

[burtenshaw]

Summary:

• Extract InspectAI eval keyword assembly into a private helper.
• Add coverage for existing max_connections and model_base_url pass-through.

Validation:

• PYTHONPATH=src:envs uv run python -m pytest tests/core/test_evals -q
• PYTHONPATH=src:envs uv run python -m pytest tests/ -q -m "not integration and not docker and not network"
• uv run ruff check src/openenv/core/evals tests/core/test_evals
• uv run ruff format --check src/openenv/core/evals tests/core/test_evals
• uv run usort check src/openenv/core/evals tests/core/test_evals