Add LangGraph integration tests

[aviralgarg05]

Summary

Add integration tests that verify AgentUnit works with real LangGraph agents.

Tasks

• Create a simple LangGraph agent in tests/integration/.
• Write integration test that runs a full evaluation cycle.
• Use pytest markers to skip if LangGraph not installed.
• Document how to run integration tests.

Acceptance criteria

• Integration tests pass with LangGraph installed.
• Tests are skipped gracefully without LangGraph.
• CI optionally runs integration tests.

[sshekhar563]

Can i work on this issue?

[aviralgarg05]

Sure, assigned!