TestBlockPostingsFormat2.testTTFBlockSize broken in branch_10x

### Description

New failure in branch_10x:
```
org.apache.lucene.backward_codecs.lucene50.TestBlockPostingsFormat2 > test suite's output saved to /opt/buildkite-agent/builds/bk-agent-prod-gcp-1777053612008421781/elastic/apache-lucene-nightly/lucene/backward-codecs/build/test-results/test/outputs/OUTPUT-org.apache.lucene.backward_codecs.lucene50.TestBlockPostingsFormat2.txt, copied below:
> java.lang.IllegalArgumentException: Document update skipped due to duplicate termdoc term
> at __randomizedtesting.SeedInfo.seed([4EBC184B6801E1FC:B6CE6240B8EE1C17]:0)
> at org.apache.lucene.core@10.5.0-SNAPSHOT/org.apache.lucene.index.IndexingChain$PerField.invertTokenStream(IndexingChain.java:1394)
...
Caused by:
> org.apache.lucene.index.TermsHashPerField$DuplicateTermException: field 'DOCS_AND_CUSTOM_FREQS' has duplicate term 'docs_and_custom_freqs'
> at app/org.apache.lucene.core@10.5.0-SNAPSHOT/org.apache.lucene.index.TermsHashPerField.add(TermsHashPerField.java:203)
> at app/org.apache.lucene.core@10.5.0-SNAPSHOT/org.apache.lucene.index.IndexingChain$PerField.invertTokenStream(IndexingChain.java:1374)
> ... 47 more
```

### Gradle command to reproduce

gradlew test --tests TestBlockPostingsFormat2.testTTFBlockSize -Dtests.seed=4EBC184B6801E1FC -Dtests.nightly=true -Dtests.locale=ff-Latn-GM -Dtests.timezone=Etc/GMT+11 -Dtests.asserts=true -Dtests.file.encoding=UTF-8

Start by running the provided Gradle command for TestBlockPostingsFormat2.testTTFBlockSize on branch_10x and read the saved test output. Inspect the test and the duplicate-term failure in the reported Lucene indexing classes. Done means the test passes with the supplied seed and nightly settings without the duplicate termdoc error.

Written by the indexing model from the issue text.