Neo4j fix

[Eric-Shang]

Why are these changes needed?

• Improve auto generating graphs
• Support connecting to existing graph
• Changed llamaIndex query engine to use conversation history internally

Related issue number

Checks

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[AgentGenie]

In the note book

Incrementally add new documents to the existing knoweldge graph!

remove !

[AgentGenie]

Did you test the code? Could you add more unit tests?

[AgentGenie]

LGTM