perf: fix N+1 query in chart list API when thumbnail_url is requested (#37895)

tests/unit_tests/thumbnails/test_digest.py

@@ -407,6 +407,7 @@ def test_chart_digest(
         **(chart_overrides or {}),
     }
     chart = Slice(**kwargs)
+    chart.table = datasource
 
     user: User | None = None
     if has_current_user:
@@ -422,12 +423,6 @@ def test_chart_digest(
                 "THUMBNAIL_CHART_DIGEST_FUNC": func,
             },
         ),
-        patch.object(
-            type(chart),
-            "datasource",
-            new_callable=PropertyMock,
-            return_value=datasource,
-        ),
         patch.object(security_manager, "find_user", return_value=user),
         override_user(user),
     ):