[Realtime API] Client-generated event_id not being persisted

Use metadata property to add your custom data like

const event = {
  type: "response.create",
  response: {
   

    // Set metadata to help identify responses sent back from the model
    metadata: { topic: "classification" },
    
    // Set any other available response fields
    modalities: [ "text" ],
    instructions: prompt,
  },
};