Implement Batched Message Storage for AgentCore Session Manager

[jariy17]

Problem

The current Strands AgentCore Session Manager calls create_event per message. Since AgentCore charges per create_event call, this results in excessive costs for customers.

Solution

Add a new input parameter to configure how many messages are batched per create_event call. Default to 1 for backwards compatibility.

Implementation Details

• Add configurable batch size parameter (default: 1)
• Add finally/close block to ensure messages are sent to failure
• Update documentation and sample code to demonstrate batching