ApichatWebhookImageMessage
- timestampType: numberrequired
Unix epoch seconds with fractional microseconds (e.g. 1780054017.039184). Not a string, not milliseconds.
- typeenumconst:imagerequiredvalues
- image
- urlType: stringFormat: urirequired
Publicly-fetchable GCS URL (…//channels/.).
- messageType: string | null
Caption; null when none. NOTE: multiple images in one block fire multiple
imagePOSTs ~30ms apart — group by customer.token + time window.