fix(backtest): remove BT-001A exact window restriction now that full data access is available

- Change WindowPolicy from EXACT to BOUNDED for backtest fixture
- Pass data_source to run_read_only_scenario so real data can be used
- Fix injected provider identity preservation in BacktestPageService
- Add type: ignore for BacktestHistoricalProvider protocol assignment
- Revert TypedDict change to avoid cascading type issues in pages/
- Update tests to reflect new BOUNDED policy behavior

app/services/backtesting/ui_service.py

@@ -105,8 +105,12 @@ class BacktestPageService:
         )
         self.template_service = template_service or base_service.template_service
         self.databento_config = databento_config
+        # Use the injected provider if available, otherwise create a new one
+        base_provider = base_service.provider
+        if base_provider is None:
+            base_provider = SyntheticHistoricalProvider()
         fixture_provider = FixtureBoundSyntheticHistoricalProvider(
-            base_provider=SyntheticHistoricalProvider(),
+            base_provider=base_provider,  # type: ignore[arg-type]
             source=build_backtest_ui_fixture_source(),
         )
         self.backtest_service = copy(base_service)