test(config): cover invalid boolean environment values

## Goal\nAdd a focused test for an invalid boolean environment value and assert the resulting configuration error identifies the variable.\n\n## Suggested files\n- tests/test_config.py, src/waggle/config.py\n\n## Acceptance criteria\n- The change stays focused on the stated scope.\n- Documentation or tests are updated as appropriate.\n- The relevant focused test or documentation validation passes.\n\nThis is intentionally scoped as a starter contribution.