2025-01-29b: New database schema for Cosmos DB

This release improves the database schema for the Cosmos DB chat history feature, based on discussions with the Cosmos DB team about avoiding excessively large document sizes. This is a breaking change, so if you already have the feature deployed, you'll need to do a full azd up to create the new container and deploy the new code, and users will not see past chat history.

If you want to migrate the past chat history to the new schema before deploying the change, please file an issue in the tracker, and we will write a migration script from the new container to the older container.

 ## What's Changed

• Improve schema of CosmosDB chat history to handle long conversations by @pamelafox in #2312

Full Changelog: 2025-01-29a...2025-01-29b