feat(swarm): Add an optional state update function to support custom state updates during handover

[KonghaYao]

When I was using the handoff feature, I found that my state.current_plan was never updating, no matter what I tried.

Initially, I thought it was my mistake and tried many different ways to set the LangGraph state, but none of them worked.

Later, I examined the source code for createHandOff and discovered that when using a Command to jump to a new Node, the state wasn't being passed along. This caused my current_plan to remain null. So, I made a modification by adding a new parameter to persist the state.

const update_plan = tool(
    async (input, config: ToolRunnableConfig) => {
        return new Command<typeof DeepResearchState.State>({
            update: {
                current_plan: input, // not working
                messages: [
                    new ToolMessage({
                        content: "完成计划更新",
                        tool_call_id: config.toolCall!.id!,
                    }),
                ],
            },
        });
    },
    {
        name: "update_plan",
        description: "Update plan for this chat",
        schema: Plan,
    }
);

const planner_agent = createReactAgent({
    name: "planner",
    llm,
    tools: [
        // SequentialThinkingTool,
        update_plan,
        createHandoffTool({ agentName: "researcher", description: "进行研究" }),
        createHandoffTool({ agentName: "reporter", description: "进行总结" }),
    ],
    prompt: async (state) => {
        const data = await apply_prompt_template("deep_research_planner.md", state as any);
        return [new SystemMessage(data), ...state.messages];
    },
    stateSchema: DeepResearchState,
});