Enabling RAG makes it relatively easy to reach 400KB limit for a dynamodb item on sessions table

[gbone-restore]

Amazon DynamoDB limits the size of each item in a table to 400 KB. It's feasible that this limit could be reached on the sessions table because of all the extra metadata stored in the item. Especially when running with RAG configured. The session table stores chat history in a single item attribute.

To work around this limitation, we can store chat history as individual items in the sessions table. I can submit a PR for this change if there is an interest in this issue.