PerFieldKnnVectorsFormat can write to wrong format name

Currently when creating a KnnVectorsWriter for merging, we consult the existing "PER_FIELD_SUFFIX_KEY" attribute to determine the format's per-field suffix. This isn't correct since we could be using a new codec (that produces different formats/ suffixes).

The attached PR modifies TestPerFieldDocValuesFormat#testMergeUsesNewFormat to trigger the problem. Without the fix we get an error like "java.nio.file.FileAlreadyExistsException: File "_3_Lucene90HnswVectorsFormat_0.vem" was already written to."