Post stores cases messages

curl --request POST \
  --url https://api.pandabase.io/v2/core/stores/{storeId}/cases/{caseId}/messages \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "body": "<string>",
  "attachments": [
    {
      "attachmentId": "<string>",
      "filename": "<string>",
      "mimeType": "<string>",
      "sizeBytes": 5242880
    }
  ]
}
'

{
  "ok": true,
  "data": {
    "id": "cmsg_q1mzj0kxw5h7n4tr3bcfp2v8",
    "senderType": "MERCHANT",
    "senderId": "cus_j8h0r1n4tzcbfp2v8q1mzwx5",
    "body": "Thanks for reaching out — looking into this now.",
    "createdAt": "2026-05-14T13:00:00.000Z",
    "attachments": []
  }
}