IndexWriter rollback deletes uncommitted segment files while pooled readers are still open

Related to #13304.

The example below can reproduce this error.

```java
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.lucene.index;

import org.apache.lucene.codecs.lucene104.Lucene104Codec;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.StringField;
import org.apache.lucene.store.ByteBuffersDirectory;
import org.apache.lucene.tests.store.MockDirectoryWrapper;
import org.apache.lucene.tests.util.LuceneTestCase;

public class TestIndexWriterRollbackReaderPool extends LuceneTestCase {

public void testRollbackUncommittedMergedSegment() throws Exception {
try (MockDirectoryWrapper dir =
new MockDirectoryWrapper(random(), new ByteBuffersDirectory())) {
dir.setAssertNoDeleteOpenFile(true);
dir.setThrottling(MockDirectoryWrapper.Throttling.NEVER);

LogDocMergePolicy mergePolicy = new LogDocMergePolicy();
mergePolicy.setMergeFactor(100);

IndexWriterConfig config =
new IndexWriterConfig()
.setCodec(new Lucene104Codec())
.setMergeScheduler(new SerialMergeScheduler())
.setMergePolicy(mergePolicy)
.setUseCompoundFile(true)
.setMaxBufferedDocs(2)
.setCommitOnClose(false);
config.getCodec().compoundFormat().setCfsThresholdDocSize(Integer.MAX_VALUE);

try (IndexWriter writer = new IndexWriter(dir, config)) {
for (int i = 0; i < 4; i++) {
Document doc = new Document();
doc.add(new StringField("id", Integer.toString(i), Field.Store.NO));
writer.addDocument(doc);
}
writer.commit();

// Commit two segments, then replace them with an uncommitted compound segment.
try (DirectoryReader committed = DirectoryReader.open(dir)) {
assertEquals(4, committed.numDocs());
assertEquals(2, committed.leaves().size());
}
writer.forceMerge(1);

try (DirectoryReader reader = DirectoryReader.open(writer)) {
assertEquals(4, reader.numDocs());
assertEquals(1, reader.leaves().size());
SegmentReader segmentReader = (SegmentReader) reader.leaves().getFirst().reader();
assertTrue(segmentReader.getSegmentInfo().info.getUseCompoundFile());
}
}

try (DirectoryReader committed = DirectoryReader.open(dir)) {
assertEquals(4, committed.numDocs());
assertEquals(2, committed.leaves().size());
}
}
}
}
```

### Fix

Simply moving the `readerPool.close()` call in `IndexWriter.rollbackInternalNoCommit()` ahead of `deleter.checkpoint(...)` is not sufficient. Although this change passes the reproducer, a full `./gradlew check` exposes `AlreadyClosedException: ReaderPool is already closed`, because the checkpoint's diagnostic logging still accesses the reader pool through `IndexWriter.segString()` and `numDeletedDocs()`.

Start with the reproducer in TestIndexWriterRollbackReaderPool.testRollbackUncommittedMergedSegment(). Read IndexWriter.rollbackInternalNoCommit(), especially the ordering of readerPool.close() and deleter.checkpoint(), then trace checkpoint diagnostics through IndexWriter.segString() and numDeletedDocs(). Done means the reproducer passes without deleting open files, and ./gradlew check completes without AlreadyClosedException.

Written by the indexing model from the issue text.