test: cover config key validation branches

[haosenwang1018]

Summary

• add coverage for set_config_value(..., project=True) writing to the project-scoped config file
• cover malformed dotted keys, unknown sections, and unknown field validation errors
• keep the coverage focused on config.py validation branches called out by issue test: improve test coverage across the project #114

Testing

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

Refs #114