Improve handling of cost_basis during holding materialization and display (#619)

- Refactored `persist_holdings` to separate and conditionally upsert holdings with and without cost_basis.
- Updated `avg_cost` logic to treat 0 cost_basis as unknown and return nil when cost_basis cannot be determined.
- Modified trend and investment calculation to exclude holdings with unknown cost_basis.
- Adjusted `average_cost` formatting to handle nil values in API responses and views.
- Added comprehensive tests to ensure cost_basis preservation and fallback behavior.
- Localized `unknown` label for display when cost_basis is unavailable.

Co-authored-by: Josh Waldrep <joshua.waldrep5+github@gmail.com>

config/locales/views/holdings/en.yml

@@ -8,6 +8,7 @@ en:
     holding:
       per_share: per share
       shares: "%{qty} shares"
+      unknown: "--"
     index:
       average_cost: Average cost
       holdings: Holdings