test: cover config dict conversion edge cases

[haosenwang1018]

Summary

• add coverage for _dict_to_config() ignoring unknown fields and unknown sections
• verify non-dict section payloads are skipped instead of crashing
• verify explicit empty section dicts still fall back to dataclass defaults

Testing

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

Refs #114