refactor(pre-alpha): fail closed on historical preview fallbacks

app/services/backtesting/ui_service.py

@@ -98,7 +98,7 @@ class BacktestPageService:
             )
         return history[0].close
 
-    def run_read_only_scenario(
+    def validate_preview_inputs(
         self,
         *,
         symbol: str,
@@ -108,7 +108,8 @@ class BacktestPageService:
         underlying_units: float,
         loan_amount: float,
         margin_call_ltv: float,
-    ) -> BacktestPageRunResult:
+        entry_spot: float | None = None,
+    ) -> float:
         normalized_symbol = symbol.strip().upper()
         if not normalized_symbol:
             raise ValueError("Symbol is required")
@@ -125,9 +126,35 @@ class BacktestPageService:
         if not template_slug:
             raise ValueError("Template selection is required")
 
+        self.template_service.get_template(template_slug)
+        resolved_entry_spot = (
+            entry_spot if entry_spot is not None else self.derive_entry_spot(normalized_symbol, start_date, end_date)
+        )
+        _validate_initial_collateral(underlying_units, resolved_entry_spot, loan_amount)
+        return resolved_entry_spot
+
+    def run_read_only_scenario(
+        self,
+        *,
+        symbol: str,
+        start_date: date,
+        end_date: date,
+        template_slug: str,
+        underlying_units: float,
+        loan_amount: float,
+        margin_call_ltv: float,
+    ) -> BacktestPageRunResult:
+        normalized_symbol = symbol.strip().upper()
+        entry_spot = self.validate_preview_inputs(
+            symbol=normalized_symbol,
+            start_date=start_date,
+            end_date=end_date,
+            template_slug=template_slug,
+            underlying_units=underlying_units,
+            loan_amount=loan_amount,
+            margin_call_ltv=margin_call_ltv,
+        )
         template = self.template_service.get_template(template_slug)
-        entry_spot = self.derive_entry_spot(normalized_symbol, start_date, end_date)
-        _validate_initial_collateral(underlying_units, entry_spot, loan_amount)
         initial_portfolio = materialize_backtest_portfolio_state(
             symbol=normalized_symbol,
             underlying_units=underlying_units,