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/pages/backtests.py

@@ -830,6 +830,7 @@ def _render_backtests_page(workspace_id: str | None = None) -> None:
                     underlying_units=units,
                     loan_amount=loan,
                     margin_call_ltv=ltv,
+                    data_source=str(data_source_select.value),
                 )
                 # Update cost in saved settings after successful run
                 if str(data_source_select.value) == "databento":