fix: switch DataService to JSON serialization to survive QuickBooks schema drift

The SDK's XML deserializer reflects on declared properties of its Data
classes and throws a RuntimeException when QuickBooks returns a field
the installed SDK version doesn't define yet (e.g. the new invoice
AllowOnlinePayPalPayment field), breaking every invoice create/update.
The JSON path builds instances from the class's own known properties
and silently ignores unrecognised response fields instead, so it
tolerates QuickBooks adding fields ahead of the SDK.

tests/ClientTest.php

@@ -1,5 +1,6 @@
 <?php
 
+use QuickBooksOnline\API\Core\Http\Serialization\SerializationFormat;
 use QuickBooksOnline\API\DataService\DataService;
 use QuickBooksOnline\API\Exception\ServiceException;
 use Wearepixel\QuickBooks\Client;
@@ -130,6 +131,27 @@
 it('returns a report service using the data service')
     ->skip('Once we figure out how to test around the static DataService::Configure');
 
+it('configures the data service to use JSON serialization so unrecognised QuickBooks response fields do not throw', function () {
+    $this->token_mock
+        ->shouldReceive('getAttribute')
+        ->twice()
+        ->with('hasValidAccessToken')
+        ->andReturnFalse();
+
+    $this->token_mock
+        ->shouldReceive('getAttribute')
+        ->once()
+        ->with('hasValidRefreshToken')
+        ->andReturnFalse();
+
+    $format = $this->client
+        ->getDataService()
+        ->getServiceContext()
+        ->IppConfiguration->Message->Request->SerializationFormat;
+
+    expect($format)->toBe(SerializationFormat::Json);
+});
+
 it('has logging off by default')
     ->skip('Have to figure out how to test this with the new code in 5.x');