Unclosed index input in TestIndexWriterWithThreads [LUCENE-9891]

This cannot be easily reporoduced form the same seed but is recurring. I think it's a bug somewhere.

```Java
> Caused by:
> java.lang.RuntimeException: unclosed IndexInput: _7_LuceneFixedGap_0.doc
> at org.apache.lucene.store.MockDirectoryWrapper.addFileHandle(MockDirectoryWrapper.java:774)
> at org.apache.lucene.store.MockDirectoryWrapper.openInput(MockDirectoryWrapper.java:826)
> at org.apache.lucene.codecs.lucene90.Lucene90PostingsReader.(Lucene90PostingsReader.java:82)
> at org.apache.lucene.codecs.blockterms.LuceneFixedGap.fieldsProducer(LuceneFixedGap.java:88)
> at org.apache.lucene.codecs.perfield.PerFieldPostingsFormat$FieldsReader.(PerFieldPostingsFormat.java:334)
> at org.apache.lucene.codecs.perfield.PerFieldPostingsFormat.fieldsProducer(PerFieldPostingsFormat.java:412)
> at org.apache.lucene.index.SegmentCoreReaders.(SegmentCoreReaders.java:117)
> at org.apache.lucene.index.SegmentReader.(SegmentReader.java:89)
> at org.apache.lucene.index.ReadersAndUpdates.getReader(ReadersAndUpdates.java:179)
> at org.apache.lucene.index.BufferedUpdatesStream$SegmentState.(BufferedUpdatesStream.java:295)
> at org.apache.lucene.index.IndexWriter.openSegmentStates(IndexWriter.java:6167)
> at org.apache.lucene.index.IndexWriter.forceApply(IndexWriter.java:5941)
> at org.apache.lucene.index.IndexWriter.tryApply(IndexWriter.java:5875)
> at org.apache.lucene.index.IndexWriter.lambda$publishFrozenUpdates$10(IndexWriter.java:2771)
> at org.apache.lucene.index.IndexWriter$EventQueue.processEventsInternal(IndexWriter.java:323)
> at org.apache.lucene.index.IndexWriter$EventQueue.processEvents(IndexWriter.java:312)
> at org.apache.lucene.index.IndexWriter.processEvents(IndexWriter.java:5718)
> at org.apache.lucene.index.IndexWriter.maybeProcessEvents(IndexWriter.java:5711)
> at org.apache.lucene.index.IndexWriter.updateDocuments(IndexWriter.java:1539)
> at org.apache.lucene.index.IndexWriter.updateDocument(IndexWriter.java:1816)
> at org.apache.lucene.index.TestIndexWriterWithThreads$IndexerThread.run(TestIndexWriterWithThreads.java:91)
```

---
Migrated from [LUCENE-9891](https://issues.apache.org/jira/browse/LUCENE-9891) by Dawid Weiss (@dweiss)

Start with TestIndexWriterWithThreads.java, especially IndexerThread.run at line 91, and trace the stack through IndexWriter, ReadersAndUpdates, SegmentReader, and the postings readers. Investigate the recurring unclosed IndexInput reported by MockDirectoryWrapper and determine a reproducible cause; done means the leak is explained and covered by a regression test.

Written by the indexing model from the issue text.