fix: correct Plex notification image lookup

Closes #5700

tests/test_plex_image_lookup.py

@@ -0,0 +1,59 @@
+import unittest
+from unittest.mock import Mock
+
+from app.modules.plex.plex import Plex
+
+
+class PlexImageLookupTest(unittest.TestCase):
+    def test_get_remote_image_by_id_uses_item_arts_for_children_key(self):
+        plex = Plex.__new__(Plex)
+        plex._host = "http://192.168.8.254:32400/"
+        plex._playhost = None
+        plex._token = "plex-token"
+        plex._plex = Mock()
+        plex._plex.fetchItems.side_effect = AssertionError("should not use raw fetchItems with /children key")
+
+        item = Mock()
+        item.TYPE = "show"
+        item.art = "/library/metadata/29242/art/1"
+        item.parentKey = None
+        item.arts.return_value = [Mock(key="https://image.tmdb.org/t/p/original/test.jpg")]
+        plex._plex.fetchItem.return_value = item
+
+        image_url = plex.get_remote_image_by_id(
+            item_id="/library/metadata/29242/children",
+            image_type="Backdrop",
+            plex_url=False,
+        )
+
+        self.assertEqual(image_url, "https://image.tmdb.org/t/p/original/test.jpg")
+        item.arts.assert_called_once_with()
+        plex._plex.fetchItem.assert_called_once_with(ekey="/library/metadata/29242/children")
+
+    def test_get_remote_image_by_id_falls_back_to_local_art_url(self):
+        plex = Plex.__new__(Plex)
+        plex._host = "http://192.168.8.254:32400/"
+        plex._playhost = None
+        plex._token = "plex-token"
+        plex._plex = Mock()
+
+        item = Mock()
+        item.TYPE = "show"
+        item.art = "/library/metadata/29242/art/1"
+        item.parentKey = None
+        item.arts.return_value = []
+        plex._plex.fetchItem.return_value = item
+
+        image_url = plex.get_remote_image_by_id(
+            item_id="/library/metadata/29242/children",
+            image_type="Backdrop",
+        )
+
+        self.assertEqual(
+            image_url,
+            "http://192.168.8.254:32400/library/metadata/29242/art/1?X-Plex-Token=plex-token",
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()