Gets the budget configuration for an agent, including current cost and usage data. Returns 404 if no budget is configured.

{
  "agentId": "string",
  "amount": 0,
  "thresholdRules": [
    {
      "percent": 0
    }
  ],
  "enforceLimit": false,
  "currentMonthlyCost": 0,
  "usagePercent": 0,
  "periodStart": 0,
  "periodEnd": 0,
  "createdAt": 0,
  "modifiedAt": 0
}