Add children media class (#39902)

Co-authored-by: Paulus Schoutsen <balloob@gmail.com>

tests/components/plex/mock_classes.py

@@ -409,6 +409,11 @@ class MockPlexLibrarySection:
         if self.title == "Photos":
             return "photo"
 
+    @property
+    def TYPE(self):
+        """Return the library type."""
+        return self.type
+
     @property
     def key(self):
         """Mock the key identifier property."""