Local clients miss batch_add_messages contract

[ehz0ah]

Problem

BaseClient declares batch_add_messages, and the LangChain chat history path uses that contract for bulk session writes. The remote HTTP client implements it, but the embedded local client and the in-memory LangChain testing client do not.

That means code paths that are valid against the remote client can fail or require special-casing when the same integration is exercised with local or in-memory clients.

Expected behavior

All OpenViking client implementations that support session message writes should satisfy the same batch_add_messages contract.

Proposed fix

• Add batch_add_messages to LocalClient.
• Add the same method to InMemoryOpenVikingClient.
• Add regression coverage for both implementations.