fix: validate nested configuration value graphs

[codeforester]

Summary

• Validate string keys throughout nested mapping/list values before merge or provenance traversal.
• Reject cycles and excessive depth/traversal with source-aware ConfigurationError messages while permitting shared aliases.
• Cover first insertion, overlays, native/attached human/JSON boundaries, cycles, and depth.

Closes #359

Validation

• uv run --extra dev --extra typer --extra quality python -m pytest tests/test_batteries_included_config.py tests/test_explicit_config_validation.py -q
• uv run --extra dev --extra typer --extra quality python -m mypy --strict lib/python/base_cli/config.py
• Ruff check/format and git diff --check