Base64-encoded data URI of the icon image (e.g. data:image/png;base64,...). Max 5MB. Used only for core_page anchors. Note the field-name asymmetry: the request field is core_page_icon (data URI input); the GET response surfaces the uploaded image as core_page_icon_url (hosted URL). Sending core_page_icon_url as a request field is silently ignored.