List rate cards.

Example response

{
  "data": [
    {
      "currency_code": "USD",
      "created_at": 1767225600,
      "updated_at": 1767225600,
      "archived_at": 1767225600,
      "rates": [
        {
          "source": "metric_units",
          "billing_mode": "last_metric_reported",
          "aggregation": "sum",
          "source_unit": "characters",
          "billing_unit": "million_characters",
          "conversion_mode": "scale",
          "invoice_group_key": "usage_credits",
          "tiers": [
            {
              "up_to": "1000000",
              "unit_amount_minor_units": 1000
            }
          ],
          "created_at": 1767225600,
          "updated_at": 1767225600,
          "archived_at": 1767225600
        }
      ]
    }
  ]
}