Enhancement: Add datetime() property to Neo4jMessageHistory

[graphadvantage]

There are no timestamps being written to the Neo4jMessageHistory (neo4j_graphrag.message_history) so it's not really that useful for message analysis, for example it's not easy to query/order recent messages.

https://neo4j.com/docs/neo4j-graphrag-python/current/_modules/neo4j_graphrag/message_history.html#Neo4jMessageHistory

Currently:

ADD_MESSAGE_QUERY = ( "MATCH (s:{node_label}) WHERE s.id = $session_id " "OPTIONAL MATCH (s)-[lm:LAST_MESSAGE]->(last_message) " "CREATE (s)-[:LAST_MESSAGE]->(new:Message) " "SET new += {{role:$role, content:$content}} " "WITH new, lm, last_message WHERE last_message IS NOT NULL " "CREATE (last_message)-[:NEXT]->(new) " "DELETE lm" )

Enhanced:

ADD_MESSAGE_QUERY = ( "MATCH (s:{node_label}) WHERE s.id = $session_id " "OPTIONAL MATCH (s)-[lm:LAST_MESSAGE]->(last_message) " "CREATE (s)-[:LAST_MESSAGE]->(new:Message) " "SET new += {{role:$role, content:$content, datetime: datetime()}} " "WITH new, lm, last_message WHERE last_message IS NOT NULL " "CREATE (last_message)-[:NEXT]->(new) " "DELETE lm" )

Thanks!
Michael