fix(pre-alpha): preserve injected provider identity

app/services/backtesting/ui_service.py

@@ -1,9 +1,11 @@
 from __future__ import annotations
 
+from copy import copy
 from dataclasses import dataclass
 from datetime import date
 from math import isclose
 
+from app.backtesting.engine import SyntheticBacktestEngine
 from app.domain.backtesting_math import materialize_backtest_portfolio_state
 from app.models.backtest import (
     BacktestRunResult,
@@ -102,10 +104,10 @@ class BacktestPageService:
         )
         self.template_service = template_service or base_service.template_service
         fixture_provider = FixtureBoundHistoricalProvider(base_service.provider)
-        self.backtest_service = BacktestService(
-            provider=fixture_provider,
-            template_service=self.template_service,
-        )
+        self.backtest_service = copy(base_service)
+        self.backtest_service.provider = fixture_provider
+        self.backtest_service.template_service = self.template_service
+        self.backtest_service.engine = SyntheticBacktestEngine(fixture_provider)
 
     def template_options(self, symbol: str = "GLD") -> list[dict[str, str | int]]:
         return [
@@ -209,7 +211,10 @@ class BacktestPageService:
             end_date=end_date,
             initial_portfolio=initial_portfolio,
             template_refs=(TemplateRef(slug=template.slug, version=template.version),),
-            provider_ref=ProviderRef(provider_id="synthetic_v1", pricing_mode="synthetic_bs_mid"),
+            provider_ref=ProviderRef(
+                provider_id=self.backtest_service.provider.provider_id,
+                pricing_mode=self.backtest_service.provider.pricing_mode,
+            ),
         )
         return BacktestPageRunResult(
             scenario=scenario,