test: cover invalid config keys

[haosenwang1018]

Summary

• add coverage for set_config_value() rejecting unknown config sections
• add coverage for set_config_value() rejecting unknown fields inside valid sections
• keep the change focused on real error branches in the shared config write path

Testing

• uv run ruff format tests/test_config.py
• uv run ruff check tests/test_config.py
• uv run python -m pytest tests/test_config.py -q

Closes #114

[zc277584121]

Closing — this is superseded by #280 which covers the same invalid key validation with a more comprehensive parametrized approach. Thanks!