Fix: change the model of DatasetVersionDiff

[ChengShi-1]

What this PR does / why we need it:

DatasetVersionDiff model, termsOfAccess should be {termsOfAccess?: changed: FieldDiff[] } from API

Previous PR Merged
197 compare dataset version #220

Related to
IQSS/dataverse-frontend#623

curl -H "X-Dataverse-key: $API_TOKEN" -X GET "$SERVER_URL/api/datasets/:persistentId/versions/2.0/compare/:draft?persistentId=$PERSISTENT_IDENTIFIER"

TermsOfAccess result could be

{
    "status": "OK",
    "data": {
        "oldVersion": {
            "versionNumber": "2.0",
            "versionState": "RELEASED",
            "lastUpdatedDate": "2025-04-02T05:35:36Z"
        },
        "newVersion": {
            "versionNumber": "DRAFT",
            "versionState": "DRAFT",
            "lastUpdatedDate": "2025-04-03T22:03:23Z"
        },
        "TermsOfAccess": {
            "changed": [
                {
                    "fieldName": "Terms of Access for Restricted Files",
                    "oldValue": "ertetre",
                    "newValue": "sgsgr"
                },
                {
                    "fieldName": "Data Access Place",
                    "oldValue": "",
                    "newValue": "qweqe"
                },
                {
                    "fieldName": "Original Archive",
                    "oldValue": "",
                    "newValue": "qwe"
                }
            ]
        }
    }
}

[ChengShi-1]

@g-saracca Hi German, there is a little tweak to DatasetVersionDiff model, in order to make this PR IQSS/dataverse-frontend#623 work. Could you help to check this?

[g-saracca]

Looks ok, pretty simple, approving 👍

[ofahimIQSS]

tests are passing - merging PR