List groups

Example response

{
  "groups": [
    {
      "id": "123e4567-e89b-12d3-a456-426614174000",
      "organization_id": "org-123",
      "name": "Engineering Team",
      "members_count": 5,
      "monthly_spend": 450.25,
      "monthly_limit": 1000,
      "created_by": "user-123",
      "created_at": "2025-01-15T10:30:00Z"
    }
  ]
}