patch: fix return type of Topic.update (#4029)

This PR fixes the return type annotation of the `update` method from
`None` to `bool`, as the method returns a boolean value indicating
whether self.values has changed

Co-authored-by: kakaogames <kakaogames@Justin-MacBook-Pro.local>

libs/langgraph/langgraph/channels/topic.py

@@ -62,7 +62,7 @@ class Topic(
                 empty.values = checkpoint
         return empty
 
-    def update(self, values: Sequence[Union[Value, list[Value]]]) -> None:
+    def update(self, values: Sequence[Union[Value, list[Value]]]) -> bool:
         current = list(self.values)
         if not self.accumulate:
             self.values = list[Value]()