Handle yfinance multiindex fallback data

[Pigbibi]

Summary

• coerce yfinance DataFrame or MultiIndex column selections to a single Series before candle conversion
• cover single-symbol MultiIndex output from yfinance in runtime adapter tests

Tests

• PYTHONPATH=. PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 /tmp/ues-test-venv/bin/python -m pytest -q tests/test_runtime_strategy_adapters.py
• ruff check application/runtime_strategy_adapters.py tests/test_runtime_strategy_adapters.py

Deployment

• Cloud Build remains the deploy owner; GitHub env-sync remains disabled and only checks its gate.