Integrate LangGraph Example

[jenna-winkler]

Description

Demonstrate LangGraph compatibility by wrapping an existing LangGraph agent with agentstack-sdk. This proves teams using LangGraph can deploy on Agent Stack infrastructure.

Acceptance criteria

• Working code in apps/agentstack-sdk-py/examples/langgraph/ wrapping LangGraph agent
• All LangGraph capabilities preserved (including graph-based orchestration patterns)
• Deploys successfully to agents.res.ibm.com
• Tutorial added to docs outlining steps taken to integrate LangGraph with Agent Stack (what needed to change, how the wrapper works, integration patterns teams should follow)

Resources

• LangGraph documentation: https://langchain-ai.github.io/langgraph/
• LangGraph examples: https://github.com/langchain-ai/langgraph/tree/main/examples

Notes

• Choose an example that stress tests our SDK rather than the easiest possible integration
• Tutorial should be detailed enough for AI tools to scrape and understand integration patterns

[xjacka]

I've created a PR with an example integration in the STORM repository: stanford-oval/storm#406