Pre-connect Realtime manager.send crashes on Omit in dict events

[KirtiRamchandani]

AsyncRealtimeConnectionManager.send() / RealtimeConnectionManager.send() call json.dumps(event) on dict payloads, but post-connect connection.send() uses maybe_transform() which strips Omit/NotGiven.

Repro:

manager = client.realtime.connect(model="gpt-4o-realtime-preview")
manager.send({"type": "response.cancel", "event_id": omit})
# TypeError: Object of type Omit is not JSON serializable

PR incoming with regression test.