Improve SSE error handling

[HeadBro1]

Summary

• Validate username before using it as an AgentScope session id.
• Return visible SSE error messages so the browser does not end with a blank response.
• Surface missing API_KEY configuration to the frontend instead of leaving the chat response blank.
• Add a lightweight /health endpoint for service checks.

Why

The current /stream handler only prints agent exceptions on the server and then closes the stream. In the browser this shows up as a generic EventSource error or blank assistant response, which makes common setup/runtime problems difficult to diagnose. The handler also accepts arbitrary username values and passes them directly to JSONSession, so session ids are not constrained before being used for persistence.

Validation

• python3 -m py_compile chatbot.py