refactor(BT-001C): share historical fixture provider

tests/test_backtest_ui.py

@@ -282,3 +282,39 @@ def test_backtest_page_service_uses_injected_provider_identity_in_provider_ref()
 
     assert result.scenario.provider_ref.provider_id == "custom_v1"
     assert result.scenario.provider_ref.pricing_mode == "custom_mode"
+
+
+def test_backtest_page_service_preserves_injected_provider_behavior_beyond_history_loading() -> None:
+    class CustomProvider(SyntheticHistoricalProvider):
+        provider_id = "custom_v2"
+        pricing_mode = "custom_mode"
+
+        def price_option_by_type(self, **kwargs: object):
+            quote = super().price_option_by_type(**kwargs)
+            return quote.__class__(
+                position_id=quote.position_id,
+                leg_id=quote.leg_id,
+                spot=quote.spot,
+                strike=quote.strike,
+                expiry=quote.expiry,
+                quantity=quote.quantity,
+                mark=42.0,
+            )
+
+    provider = CustomProvider(source=StaticBacktestSource(), implied_volatility=0.2, risk_free_rate=0.01)
+    injected_service = BacktestService(provider=provider)
+    page_service = BacktestPageService(backtest_service=injected_service)
+
+    result = page_service.run_read_only_scenario(
+        symbol="GLD",
+        start_date=date(2024, 1, 2),
+        end_date=date(2024, 1, 8),
+        template_slug="protective-put-atm-12m",
+        underlying_units=1000.0,
+        loan_amount=68000.0,
+        margin_call_ltv=0.75,
+    )
+
+    template_result = result.run_result.template_results[0]
+    assert template_result.daily_path[0].option_market_value == 42000.0
+    assert template_result.summary_metrics.total_hedge_cost == 42000.0