refactor(pre-alpha): fail closed on historical fixture bounds

tests/test_backtest_ui.py

@@ -150,3 +150,38 @@ def test_backtest_page_service_validation_errors_are_user_facing(kwargs: dict[st
 
     with pytest.raises(ValueError, match=message):
         service.run_read_only_scenario(**kwargs)
+
+
+def test_backtest_page_service_fails_closed_outside_seeded_fixture_window() -> None:
+    service = BacktestPageService()
+
+    with pytest.raises(ValueError, match="deterministic fixture data only supports GLD"):
+        service.derive_entry_spot("GLD", date(2024, 1, 3), date(2024, 1, 8))
+
+    with pytest.raises(ValueError, match="deterministic fixture data only supports GLD"):
+        service.run_read_only_scenario(
+            symbol="GLD",
+            start_date=date(2024, 1, 3),
+            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,
+        )
+
+
+def test_backtest_preview_validation_reuses_supplied_entry_spot() -> None:
+    service = BacktestPageService()
+
+    entry_spot = service.validate_preview_inputs(
+        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,
+        entry_spot=100.0,
+    )
+
+    assert entry_spot == 100.0