perf(event): add status event into the agent event system

**Background**

We have established an event system to support frontend interactions and human-in-the-loop (HITL) workflows. Currently, when the agent performs time-consuming internal operations — such as compacting the context — there is no way to notify the frontend, leaving users uninformed about what is happening in the background.

**Changes**

- Introduced a general-purpose status event mechanism that allows the backend to emit status notifications to the frontend during time-consuming operations.
- These events are designed to be operation-agnostic, enabling users to customize the event content rather than being tied to any specific operation, ensuring flexibility and extensibility for future use cases.