Update ReadOnlyEngine inside of resetEngineToGlobalCheckpoint to impl…

…ement getSegmentInfosSnapshot.

This ensures access to this function is not permitted on the ReadOnlyEngine and is delegated to the new IE once opened.

Signed-off-by: Marc Handalian <handalm@amazon.com>

server/src/main/java/org/opensearch/index/shard/IndexShard.java

@@ -4582,6 +4582,16 @@ public GatedCloseable<IndexCommit> acquireSafeIndexCommit() {
                     }
                 }
 
+                @Override
+                public GatedCloseable<SegmentInfos> getSegmentInfosSnapshot() {
+                    synchronized (engineMutex) {
+                        if (newEngineReference.get() == null) {
+                            throw new AlreadyClosedException("engine was closed");
+                        }
+                        return newEngineReference.get().getSegmentInfosSnapshot();
+                    }
+                }
+
                 @Override
                 public void close() throws IOException {
                     assert Thread.holdsLock(engineMutex);