fix: use timezone-aware websocket timestamps

[Ghraven]

Summary

• Replace datetime.utcnow() in AutoGen Studio websocket responses with timezone-aware UTC timestamps
• Keep websocket error/pong payload timestamps explicit about UTC

Problem

datetime.utcnow() returns a naive datetime and is deprecated in Python 3.12. These websocket payloads are timestamped as UTC values, so using datetime.now(timezone.utc) makes that timezone explicit in the serialized ISO string.

Before / After

Before:

datetime.utcnow().isoformat()

After:

datetime.now(timezone.utc).isoformat()

Verification

• python -m compileall -q python\packages\autogen-studio\autogenstudio\web\routes\ws.py
• git diff --check

The local sparse checkout had Windows long-path limitations for the full repository, so I verified the touched Python file directly.