Improve backtest lazy loading and test automation

tests/test_backtest_ui.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import os
 from datetime import date
 from decimal import Decimal
 
@@ -175,6 +176,49 @@ def test_backtest_page_service_validation_errors_are_user_facing(kwargs: dict[st
         service.run_read_only_scenario(**kwargs)
 
 
+def test_backtest_page_service_fails_closed_for_unsupported_databento_gc_symbol() -> None:
+    service = BacktestPageService()
+
+    with pytest.raises(ValueError, match="Databento backtests currently support GLD and XAU only"):
+        service.validate_preview_inputs(
+            symbol="GC",
+            start_date=date(2024, 7, 1),
+            end_date=date(2024, 7, 5),
+            template_slug="protective-put-atm-12m",
+            underlying_units=1000.0,
+            loan_amount=68000.0,
+            margin_call_ltv=0.75,
+            data_source="databento",
+        )
+
+
+def test_backtest_page_service_allows_databento_xau_proxy_symbol() -> None:
+    service = BacktestPageService()
+
+    # Support validation should pass before the provider is consulted.
+    service.validate_data_source_support("XAU", "databento")
+
+
+@pytest.mark.skipif(not os.getenv("DATABENTO_API_KEY"), reason="requires DATABENTO_API_KEY")
+def test_backtest_page_service_runs_live_databento_gld_scenario() -> None:
+    service = BacktestPageService()
+
+    result = service.run_read_only_scenario(
+        symbol="GLD",
+        start_date=date(2024, 7, 1),
+        end_date=date(2024, 7, 5),
+        template_slug="protective-put-atm-12m",
+        underlying_units=1000.0,
+        loan_amount=68000.0,
+        margin_call_ltv=0.75,
+        data_source="databento",
+    )
+
+    assert result.entry_spot > 0
+    assert len(result.run_result.template_results[0].daily_path) >= 4
+    assert result.data_cost_usd >= 0
+
+
 def test_backtest_page_service_fails_closed_outside_seeded_fixture_window() -> None:
     """Test that fixture data fails for dates outside the seeded window."""
     service = BacktestPageService()