test: cover nested env-ref dict resolution

[haosenwang1018]

Summary

• add focused coverage for _resolve_env_refs_in_dict() resolving nested env: string values
• verify non-string values like bools and ints pass through unchanged
• keep the patch scoped to config helper branches under 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
• uv run ruff format --check tests/test_config.py

Refs #114

[zc277584121]

This PR has merge conflicts after recent merges to main (multiple PRs added tests to the same region of test_config.py). Could you rebase on latest main to resolve? Thanks!