test: add config env resolution tests

[haosenwang1018]

Adds config environment variable resolution tests (#114). All 6 tests passed. Part of #114

[zc277584121]

There's a correctness issue: test_resolve_unknown_env_var expects resolve_env_ref("env:UNKNOWN_VAR_XYZ") to return the string as-is, but the actual code raises KeyError when the env var is not set (see config.py:102). This test would fail. It should use pytest.raises(KeyError) instead.

Also test_resolve_none passes None to a function typed as str — not a meaningful test case.

Please remove the duplicate test_cli_help.py as well.