GH-51626: [C++][Parquet] Fix statistics of dictionary-encoded leaves under null parents

[kita-renji]

Rationale for this change

WriteArrowDictionary can write wrong Parquet statistics for dictionary<*, string|binary> columns (see #51626):

1. Statistics are computed from the leaf indices before their validity is replaced with the one derived from the def levels. An index under a null parent (e.g. a null struct row) can be valid in the leaf array, so it is counted as a value and its dictionary value can become min/max.
2. Unique() returns one null entry when a batch has null indices, so the shortcut that re-uses the whole dictionary for min/max fires when exactly one dictionary entry is unreferenced, and that entry can become min/max.

min/max are marked exact, and readers use them: DuckDB drops rows for s.x IS NULL and miscounts count(s.x), DataFusion returns a value that isn't in the data for min(c), max(c).

What changes are included in this PR?

• Call MaybeReplaceValidity before update_stats in WriteIndicesChunk, so statistics see the same validity as the encoded data. This is the order WriteArrowDense already uses.
• Don't count the null entry of Unique() when checking whether all dictionary entries are referenced.

Both are needed: with only the first change, the slots under null parents become null indices and case 2 puts the hidden value back into min/max.

Are these changes tested?

Yes. Two cases added to the NoNullCountWrittenForRepeatedFields (PARQUET-2067) suite in arrow_statistics_test.cc: a struct<dictionary<int32, utf8>> with null rows over valid child slots, and a top-level dictionary with a null and one unreferenced entry. Both fail without the change. All parquet C++ test binaries pass.

I also ran a randomized check (not included) that writes dictionary<string/binary> leaves nested in struct/list/large_list/fixed_size_list (up to two levels), with nulls at every level, values hidden under null parents, slices, several batches, small pages and row groups, dictionary fallback and V1/V2 pages, and compares the written statistics with ones computed from the logical values. On main it finds wrong statistics in most nested shapes (and trips the V2 DCHECK); with this change 4500 random cases match.

Are there any user-facing changes?

Files written with dictionary encoding get correct null_count and min/max for these columns. No API changes.

This PR contains a "Critical Fix". It fixes a bug that writes incorrect statistics, which makes other readers return wrong results. The data itself was always written correctly.

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• GitHub Issue: [C++][Parquet] Wrong statistics for dictionary-encoded string columns: nulls under a null struct are not counted, unused dictionary values end up in min/max #51626

[github-actions]

⚠️ GitHub issue #51626 has been automatically assigned in GitHub to PR creator.