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.

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• Realtime: update model to have a single send_event method #1111
• -> Realtime: forward exceptions from transport layer #1107
• Run make format #1106
• Realtime: run guardrails without blockign event loop #1104