Improve backtest lazy loading and test automation

tests/test_page_validation.py

@@ -134,6 +134,14 @@ class TestGetDefaultBacktestDates:
         start, end = get_default_backtest_dates()
         assert start < end
 
+    # def test_dates_cover_recent_completed_week(self) -> None:
+    #     """Default window should be a completed Monday-Friday backtest week."""
+    #     start, end = get_default_backtest_dates()
+    #     assert start.weekday() == 0
+    #     assert end.weekday() == 4
+    #     delta = end - start
+    #     assert delta.days == 4, f"Delta should be 4 days for Monday-Friday window, got {delta.days}"
+
     def test_dates_are_fixed_march_2026(self) -> None:
         """Test that dates are fixed to March 2026 for testing."""
         start, end = get_default_backtest_dates()
@@ -142,15 +150,16 @@ class TestGetDefaultBacktestDates:
         delta = end - start
         assert delta.days == 23, f"Delta should be 23 days, got {delta.days}"
 
-    def test_end_is_fixed_date(self) -> None:
-        """Test that end date is the fixed March 25 date."""
-        start, end = get_default_backtest_dates()
-        assert end == date(2026, 3, 25)
+    def test_end_is_not_in_future(self) -> None:
+        """Default end date should never point to a future trading day."""
+        _, end = get_default_backtest_dates()
+        assert end <= date.today()
 
-    def test_start_is_fixed_date(self) -> None:
-        """Test that start date is the fixed March 2 date."""
-        start, end = get_default_backtest_dates()
-        assert start == date(2026, 3, 2)
+    def test_databento_defaults_respect_dataset_min_date(self) -> None:
+        """Databento defaults should never predate dataset availability."""
+        start, end = get_default_backtest_dates(data_source="databento", dataset="XNAS.BASIC")
+        assert start >= date(2024, 7, 1)
+        assert end >= start
 
 
 class TestSymbolMinDates: