fix(langgraph): guard replace_state() against closed StateManager in execute()

Replace direct state assignment in LangGraph.execute() with
state_manager.replace_state(), which enforces the is_closed guard and
notifies state stream subscribers through the proper StateManager API,
preventing silent state corruption after StateManager.close() is called.

replace_state() is the semantically correct method for this use case:
it atomically replaces the entire state for a fresh execution context,
enforces the is_closed guard, and notifies subscribers. update_state()
is designed for incremental updates with execution_count tracking, not
for resetting state to a fresh execution context.

Closes #9994

src/cleveragents/langgraph/graph.py

@@ -108,7 +108,9 @@ class LangGraph:  # pylint: disable=too-many-instance-attributes
             if isinstance(input_data, GraphState)
             else GraphState.from_dict(input_data)
         )
-        # Replace state manager state for a fresh execution context
+        # Use replace_state() so the is_closed guard is enforced and state stream
+        # subscribers are notified through the proper StateManager API, preventing
+        # silent state corruption after StateManager.close() has been called.
         self.state_manager.replace_state(state)
         start_stream = f"__{self.name}_node_start__"
         if start_stream in self.stream_router.streams: