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]
 

tests/test_backtesting_fixture_source.py

@@ -54,14 +54,14 @@ def test_shared_fixture_source_bounded_window_allows_subranges_but_fails_closed_
         source.load_daily_closes("GLD", date(2024, 1, 5), date(2024, 1, 3))
 
 
-def test_backtest_page_service_uses_shared_exact_fixture_source() -> None:
+def test_backtest_page_service_uses_shared_bounded_fixture_source() -> None:
     service = BacktestPageService()
 
     source = service.backtest_service.provider.source
 
     assert isinstance(source, SharedHistoricalFixtureSource)
     assert source.feature_label == "BT-001A"
-    assert source.window_policy is WindowPolicy.EXACT
+    assert source.window_policy is WindowPolicy.BOUNDED
 
 
 def test_event_comparison_page_service_uses_shared_bounded_fixture_source() -> None: