[cangen185-sudo]

Summary

An existing config.yaml containing a YAML list, string, boolean, or number can abort runtime environment assembly with AttributeError in flatten_dict(). For example, - item reaches .items() even though the compatibility loader expects a mapping.

Validate the root before flattening and follow the loader's existing warning-and-skip behavior for invalid configuration. Other environment sources continue to merge normally. Valid mappings and environment precedence are unchanged.

Validation

• 51 offline tests passed: python -m pytest tests/toolkit/config tests/client/test_base_service_client_errors.py tests/client/test_tools_mcp_types.py tests/client/test_tools_tos_mount_types.py tests/utils -q.
• The five new regression cases failed on unchanged production code; after the fix they pass, including retention of .env, common, and strategy values.
• Ruff checks pass for the changed tests; formatting checks pass for both files. The source file has the same 11 existing lint findings before and after this patch, with no new findings.
• Wheel and source distribution build passed; git diff --check passed.
• Windows 11, CPython 3.12.14. Cloud deployment, Docker integration, and real account calls were not run.

Prepared with Codex assistance. A separate read-only review checked the root cause and final diff. No dependencies or generated files changed.