[Feat]: Streaming mode append msg

### Is your feature request related to a problem? Please describe.

When I use streaming mode, the agent's return value is split into N parts, which is not user-friendly in terms of interaction experience.

### Describe the solution you'd like

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look for the streaming mode implementation in the frontend code, likely in a component handling agent responses. The screenshot suggests appending messages instead of splitting them. Find where the streamed chunks are currently rendered and modify to accumulate and display as a single message. Check for existing tests related to streaming or agent output.

Scritto dal modello di indicizzazione a partire dal testo della issue.