fix: add IVF_ON_DISK case to VectorIndexUtils.getIndexFileExtension() (#18176)

IVF_ON_DISK segments failed to load on the server with
"Unsupported vector backend type: IVF_ON_DISK" because
getIndexFileExtension() had no case for IVF_ON_DISK.

IVF_ON_DISK reuses the IVF_FLAT binary file format at build time
and uses FileChannel random-access reads at query time, so it
should return VECTOR_IVF_FLAT_INDEX_FILE_EXTENSION, consistent
with how SegmentDirectoryPaths.findVectorIndexIndexFile() already
handles IVF_ON_DISK.

Author: xiangfu0

pinot-segment-local/src/main/java/org/apache/pinot/segment/local/segment/store/VectorIndexUtils.java

@@ -98,6 +98,9 @@ public static String getIndexFileExtension(VectorBackendType backendType) {
         return Indexes.VECTOR_IVF_FLAT_INDEX_FILE_EXTENSION;
       case IVF_PQ:
         return Indexes.VECTOR_IVF_PQ_INDEX_FILE_EXTENSION;
+      case IVF_ON_DISK:
+        // IVF_ON_DISK reuses the IVF_FLAT file format with FileChannel random-access reads
+        return Indexes.VECTOR_IVF_FLAT_INDEX_FILE_EXTENSION;
       default:
         throw new IllegalStateException("Unsupported vector backend type: " + backendType);
     }