Portfolio currency conversion can construct NAN=X and break recordings

[jcoffi]

Summary
Currency conversion within portfolio collection/performance attempts to fetch NAN=X on Yahoo, leading to 404s and blank Current Value/Return/Alpha fields in recorded CSVs.

Steps to reproduce

1. Run: pytest -q tests/portfolio/test_portfolio_controller.py::test_collect_historical_data
2. Run: pytest -q tests/portfolio/test_portfolio_controller.py::test_get_transactions_performance

Observed behavior

• yfinance logs: HTTP Error 404: Quote not found for symbol: NAN=X
• Resulting CSVs have blank Current Value/Return/Alpha where recordings expect values.

Expected behavior

• Currency conversion is skipped or guarded when the portfolio has no explicit currency column, and no NAN=X symbol is constructed.

Context

• The tests set portfolio._currency_column = None to avoid currency conversion.
• Currency conversion logic constructs pairs from historical statistics currency vs transaction currency; when a currency is missing/NaN this results in NAN=X and a 404.

Environment

• Branch: main (upstream)
• Tests use recorded datasets in tests/datasets without external credentials

Potential approaches (for maintainers’ consideration)

• Add guards to skip conversion when currency codes are missing/NaN or when _currency_column is falsy
• Validate derived currency pair strings before requesting market data

Additional information

• This was encountered while running the full suite; unrelated to feature work. Reporting for visibility and guidance on intended behavior when currency codes are absent.