Implement Batched Message Storage for AgentCore Session Manager

## 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

Locate the AgentCore Session Manager implementation and its create_event call, then read the surrounding message lifecycle before deciding where batching and close/finally handling belong. Update the documented parameter and sample code, and verify that the default remains one message per call while pending messages are sent on failure or close.

Written by the indexing model from the issue text.