fix(showcase/langgraph-typescript): regenerate agent package-lock.json (CopilotKit#4815)

## Summary

Regenerates `src/agent/package-lock.json` for the langgraph-typescript
integration. 5 dependencies drifted after multiple package.json bumps
without regenerating the nested lock file, causing `npm ci` to fail
in Docker builds.

| Dependency | package.json | package-lock.json (was) |
|---|---|---|
| @copilotkit/sdk-js | 1.56.5 | 1.51.4 |
| @langchain/core | 1.1.44 | ^1.0.1 |
| @langchain/langgraph | 1.3.0 | 1.0.2 |
| @langchain/openai | 1.4.4 | ^1.1.3 |
| langchain | 1.3.4 | ^1.3.3 |

## Test plan

- [ ] langgraph-typescript build-check passes in CI

Author: jpr5