.NET: Cosmos DB Store Implementations #1396

Summary

The framework has solid abstractions for storage (ChatMessageStore, ICheckpointStore<T>) with in-memory and file system implementations, plus Azure AI Agents service integration. However, there's no direct Cosmos DB support for self-managed scenarios.

What's Needed

Concrete Cosmos DB implementations of the existing storage abstractions. This would include a Cosmos DB-backed chat message store that extends ChatMessageStore, a checkpoint store implementing ICheckpointStore<T> for workflow state, and support for thread persistence using the existing thread serialization capabilities.

Implementation

Should follow existing factory patterns and use Cosmos DB best practices for partitioning and indexing. Integration with the framework's existing serialization patterns is essential.