fix(AlertsReports): validate anchor_list is a list (#38723)

superset/commands/report/execute.py

@@ -268,7 +268,11 @@ class BaseReportState:
             native_filter_params = self._report_schedule.get_native_filters_params()
             if anchor := dashboard_state.get("anchor"):
                 try:
-                    anchor_list: list[str] = json.loads(anchor)
+                    anchor_list = json.loads(anchor)
+                    if not isinstance(anchor_list, list):
+                        raise json.JSONDecodeError(
+                            "Anchor value is not a list", anchor, 0
+                        )
                     urls = self._get_tabs_urls(
                         anchor_list,
                         native_filter_params=native_filter_params,

tests/unit_tests/commands/report/execute_test.py

@@ -259,6 +259,12 @@ def test_log_data_with_missing_values(mocker: MockerFixture) -> None:
             ["url1"],
             ["superset/dashboard/p/url1/"],
         ),
+        # Test JSON scalar string anchor falls back to single tab
+        (
+            json.dumps("mock_tab_anchor_1"),
+            ["url1"],
+            ["superset/dashboard/p/url1/"],
+        ),
     ],
 )
 @patch(