EventBridge rule trigger failing with 403 on Genesys Cloud webhook payload

Dealing with a very strange bug here with my AWS EventBridge integration. I set up an event bus and rule to capture Genesys Cloud interaction events, but the target Lambda keeps rejecting the payload. The EventBridge rule matches the source genesyscloud, yet the Lambda handler throws a validation error on the event structure.

Here is the minimal Deno-style fetch test I used to verify the webhook endpoint is alive:

const res = await fetch('https://my-lambda-url.execute-api.ap-northeast-2.amazonaws.com', {
 method: 'POST',
 headers: { 'Content-Type': 'application/json' },
 body: JSON.stringify({ detail: { event: 'interaction.created' } })
});
console.log(res.status); // Returns 200

However, when Genesys Cloud actually pushes the event via EventBridge, the Lambda receives a malformed object. The detail field is missing the nested conversationId. Is there a specific JSON schema I need to enforce in the EventBridge target input transformer, or is Genesys Cloud sending a different envelope format than the standard EventBridge spec?