feat(service): add new endpoint to expose the background tasks

**Background**

Background execution of timed-out tools is currently supported. However, there is no way for the frontend or users to observe the status of these background tasks, leaving them unaware of ongoing executions.

To address this, a new endpoint should be introduced to expose background task information for frontend display.

**Changes**

Considering that most tools are non-streaming, surfacing their intermediate output in real time provides little value. Therefore, the initial approach should focus on **listing background tasks** on the frontend, allowing users to track task status without overwhelming them with verbose, incremental results.