[Agent Feature] SSE robustness and Store abstraction

**What would you like to be added**:
Agent SSE 健壮性，Store的抽象（先把现有实现作为MemoryStore）
**Why is this needed**:

Review the existing Agent SSE implementation and the current Store implementation first. Clarify the expected robustness behavior and the abstraction boundary before changing anything; done should include a Store interface with the existing implementation treated as MemoryStore and reliable Agent SSE behavior.

Written by the indexing model from the issue text.