fix: preserve graph-owned state keys during frontend state merge (CopilotKit#3852)

## Summary
- During frontend state merge in LangGraph agent, preserve keys that are
owned by the graph (not set by the frontend)
- Prevents graph-computed state from being overwritten when frontend
state is merged back

Closes CopilotKit#2893

---
*Split from CopilotKit#3847*

Author: jpr5

sdk-python/copilotkit/langgraph_agent.py

@@ -267,7 +267,12 @@ async def prepare_stream( # pylint: disable=too-many-arguments
             actions=actions,
             agent_name=self.name
         )
-        current_graph_state.update(state)
+        # Only update graph state with keys that merge_state explicitly produced,
+        # not keys that were simply passed through from state_input unchanged.
+        # This preserves graph-owned state keys that the frontend may have sent stale values for.
+        for key, value in state.items():
+            if key not in state_input or value is not state_input.get(key):
+                current_graph_state[key] = value
         lg_interrupt_meta_event = next((ev for ev in (meta_events or []) if ev.get("name") == "LangGraphInterruptEvent"), None)
         has_active_interrupts = active_interrupts is not None and len(active_interrupts) > 0