perf(event): improve the current event system

**Background**
We have established an event system to support frontend interactions and human-in-the-loop (HITL) workflows. However, there is room for improvement in handling loop limits and user interventions.

**Changes**

- **Iteration extension events**
- Added `RequireIterationExtensionEvent` to be triggered when the execution/iteration loop limit is exceeded.
- Added a corresponding `IterationExtensionResultEvent` to capture and process user confirmation results.

- **General status events**: When the agent performs time-consuming operations — such as compacting the context — the backend should emit corresponding events to notify the frontend and keep the user informed. These events should be designed as a general-purpose mechanism, allowing users to customize the event content rather than being tied to a specific operation.