Realtime: forward exceptions from transport layer

[rm-openai]

The model listens to websocket events in a separate task, and the session receives those events in on_event. So if there's e.g. a websocket or json exception, it's silently dropped. Similar to the streaming text agents, we should:

• capture exceptions separately
• raise them in async for event in session loop.

Added tests to verify.

---

• Realtime: send session.update event at connection/handoffs #1112
• Realtime: update model to have a single send_event method #1111
• -> Realtime: forward exceptions from transport layer #1107
• Run make format #1106