Mark message processing as complete

curl --request POST \
  --url http://localhost:3000/api/messaging/complete \
  --header 'Content-Type: application/json' \
  --data '{
  "agentId": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "roomId": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "userId": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "prompt": "<string>"
}'

{
  "success": true,
  "data": {
    "response": "<string>"
  }
}