Realtime: run guardrails without blockign event loop

[rm-openai]

Guardrails were blocking the event loop. Now, they run as a separate task.

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• 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
• -> Realtime: run guardrails without blockign event loop #1104