store: Clamp adaptive batch size for stale cleanup

[mk0walsk]

Summary

Clamp adaptive batch sizes so stale call cache cleanup never degrades to a zero-sized batch.

Changes

• clamp AdaptiveBatchSize to a minimum of 1 when it is created, adapted, or set explicitly
• guard clear_stale_call_cache before issuing each delete batch so the loop always makes progress
• add a unit test covering the zero-sized batch regression path