`PurchaseEx` field is not populated when fetching Purchase objects

## Description
I am trying to query and retrieve Purchase objects from the QuickBooks Online API. While the REST API response correctly includes the `PurchaseEx` field, the corresponding property in the `Purchase` object within the .NET SDK is always null.

This issue prevents me from accessing critical data (specifically custom fields) that is essential for my application's functionality.

## Steps to Reproduce
- Use the DataService to query for Purchase objects.
- Retrieve a Purchase object from the query result.
- Inspect the PurchaseEx property on the object.
**Expected behavior:** The PurchaseEx property is populated with data.
**Actual behavior:** The PurchaseEx property is null.

## Proofs

When using the SDK

When using the API

```JSON
{
"QueryResponse": {
"Purchase": [
{
"AccountRef": {
"value": "41",
"name": "Mastercard"
},
"PaymentType": "CreditCard",
"Credit": false,
"TotalAmt": 34,
"PurchaseEx": {
"any": [
{
"name": "{http://schema.intuit.com/finance/v3}NameValue",
"declaredType": "com.intuit.schema.finance.v3.NameValue",
"scope": "javax.xml.bind.JAXBElement$GlobalScope",
"value": {
"Name": "TxnType",
"Value": "1"
},
"nil": false,
"globalScope": true,
"typeSubstituted": false
}
]
},
"domain": "QBO",
"sparse": false,
"Id": "144",
"SyncToken": "0",
"MetaData": {
"CreateTime": "2025-04-27T14:04:25-07:00",
"LastUpdatedTime": "2025-04-27T14:04:25-07:00"
},
"TxnDate": "2025-04-27",
"CurrencyRef": {
"value": "USD",
"name": "United States Dollar"
},
"Line": [
{
"Id": "1",
"Amount": 34,
"DetailType": "AccountBasedExpenseLineDetail",
"AccountBasedExpenseLineDetail": {
"AccountRef": {
"value": "55",
"name": "Automobile"
},
"BillableStatus": "NotBillable",
"TaxCodeRef": {
"value": "NON"
}
}
}
]
},
{
"AccountRef": {
"value": "41",
"name": "Mastercard"
},
"PaymentType": "CreditCard",
"EntityRef": {
"value": "41",
"name": "Hicks Hardware",
"type": "Vendor"
},
"Credit": false,
"TotalAmt": 42.4,
"PurchaseEx": {
"any": [
{
"name": "{http://schema.intuit.com/finance/v3}NameValue",
"declaredType": "com.intuit.schema.finance.v3.NameValue",
"scope": "javax.xml.bind.JAXBElement$GlobalScope",
"value": {
"Name": "TxnType",
"Value": "1"
},
"nil": false,
"globalScope": true,
"typeSubstituted": false
}
]
},
"domain": "QBO",
"sparse": false,
"Id": "143",
"SyncToken": "0",
"MetaData": {
"CreateTime": "2025-04-16T14:45:20-07:00",
"LastUpdatedTime": "2025-04-16T14:45:20-07:00"
},
"TxnDate": "2025-04-08",
"CurrencyRef": {
"value": "USD",
"name": "United States Dollar"
},
"Line": [
{
"Id": "1",
"Amount": 42.4,
"DetailType": "AccountBasedExpenseLineDetail",
"AccountBasedExpenseLineDetail": {
"AccountRef": {
"value": "64",
"name": "Decks and Patios"
},
"BillableStatus": "NotBillable",
"TaxCodeRef": {
"value": "NON"
}
}
}
]
}
],
"startPosition": 1,
"maxResults": 2
},
"time": "2025-08-28T00:45:21.467-07:00"
}
```