Adjusting PR #15415 for backport

Author: benwtrent

lucene/core/src/test/org/apache/lucene/codecs/lucene104/TestLucene104ScalarQuantizedVectorsFormat.java

@@ -33,6 +33,7 @@
 import org.apache.lucene.codecs.KnnVectorsReader;
 import org.apache.lucene.codecs.lucene104.Lucene104ScalarQuantizedVectorsFormat.ScalarEncoding;
 import org.apache.lucene.codecs.lucene95.OrdToDocDISIReaderConfiguration;
+import org.apache.lucene.codecs.perfield.PerFieldKnnVectorsFormat;
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.KnnFloatVectorField;
 import org.apache.lucene.index.CodecReader;
@@ -271,7 +272,9 @@ public void testReadQuantizedVectorWithEmptyRawVectors() throws Exception {
         LeafReader r = getOnlyLeafReader(reader);
         if (r instanceof CodecReader codecReader) {
           KnnVectorsReader knnVectorsReader = codecReader.getVectorReader();
-          knnVectorsReader = knnVectorsReader.unwrapReaderForField(vectorFieldName);
+          if (knnVectorsReader instanceof PerFieldKnnVectorsFormat.FieldsReader fieldsReader) {
+            knnVectorsReader = fieldsReader.getFieldReader(vectorFieldName);
+          }
           if (knnVectorsReader instanceof Lucene104ScalarQuantizedVectorsReader quantizedReader) {
             FloatVectorValues floatVectorValues =
                 quantizedReader.getFloatVectorValues(vectorFieldName);