Get all NI categories for an employee

curl --request GET \
  --url https://api.sandbox.flowpayroll.ai/v1/employees/{employeeId}/ni-categories \
  --header 'X-Auth-Token: <api-key>'

{
  "message": {
    "text": "<string>",
    "token": "<string>",
    "tokenArguments": [
      {
        "name": "<string>",
        "value": "<string>"
      }
    ]
  },
  "content": {
    "data": {
      "categoriesPerYear": {
        "2024": [
          {
            "category": "A",
            "effectiveDate": "2024-04-06",
            "reason": "Annual Review",
            "issuedBy": "HMRC"
          }
        ],
        "2025": [
          {
            "category": "A",
            "effectiveDate": "2025-04-06",
            "reason": "Annual Review",
            "issuedBy": "HMRC"
          }
        ]
      }
    },
    "metadata": {
      "dateFormat": "yyyy-MM-dd",
      "dateTimeFormat": "yyyy-MM-ddTHH:mm:ss.fffZ",
      "paginationToken": "<string>"
    }
  },
  "validationIssues": [
    {
      "field": "<string>",
      "reason": "<string>",
      "reasonToken": "<string>",
      "reasonTokenArguments": [
        {
          "name": "<string>",
          "value": "<string>"
        }
      ]
    }
  ],
  "messageToken": "<string>"
}