List providers

curl --request GET \
  --url https://app.sendmux.ai/api/v1/providers \
  --header 'Authorization: Bearer <token>'

{
  "ok": true,
  "data": [
    {
      "id": "dprov_clxxxxxxxxxxxxxxxxxxxxxxxxx",
      "name": "Gmail API - Marketing",
      "type": "smtp",
      "status": "active",
      "status_reason": "<string>",
      "is_active": true,
      "from_email": "<string>",
      "from_name": "<string>",
      "created_at": "2026-03-01T10:00:00Z",
      "last_tested_at": "<string>",
      "last_used_at": "<string>"
    }
  ],
  "meta": {
    "request_id": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx"
  }
}