test: cover CLI config helper mappings

[haosenwang1018]

Summary

• add direct unit coverage for _build_cli_overrides()
• verify the shared CLI override mapping only includes non-None values and lands in the expected nested config sections
• add focused coverage for _cfg_to_memsearch_kwargs() so the resolved config -> MemSearch(...) bridge stays stable

Testing

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

Closes #114