Check if a subscription exists for a given webhook and user

curl --request GET \
  --url https://api.twitter.com/2/account_activity/webhooks/{webhook_id}/subscriptions/all \
  --header 'Authorization: Bearer <token>'

{
  "data": {
    "subscribed": true
  },
  "errors": [
    {
      "detail": "<string>",
      "status": 123,
      "title": "<string>",
      "type": "<string>"
    }
  ]
}