fix(backtest): improve error message for dates outside fixture window

- Add helpful message suggesting Databento/Yahoo Finance for dates outside fixture range
- Update test to expect BOUNDED policy for backtest UI

app/services/backtesting/fixture_source.py

@@ -54,7 +54,8 @@ class SharedHistoricalFixtureSource:
             if start_date < self.start_date or end_date > self.end_date:
                 raise ValueError(
                     f"{self.feature_label} deterministic fixture data only supports the seeded "
-                    f"{self.start_date.isoformat()} through {self.end_date.isoformat()} window"
+                    f"{self.start_date.isoformat()} through {self.end_date.isoformat()} window. "
+                    f"For dates outside this range, please use Databento or Yahoo Finance data source."
                 )
         return [point for point in self.history if start_date <= point.date <= end_date]