fix: treat missing parquet null counts as zero only for legacy writer…

[Cherylgabba]

Which issue does this PR close?

Closes #25253

Rationale for this change

parquet-rs < 53.1.0 (e.g. the version linked by DataFusion < 42.1.0) did not record a null_count in row-group statistics when it was zero. Treating a missing null_count as exactly zero is therefore a safe optimization for those files — but it is wrong for every other writer that genuinely omits null counts. Assuming zero for those files lets IS NULL / COUNT row-group pruning, file statistics, limit pruning and sort pushdown return incorrect results.

What changes are included in this PR?

• Added missing_null_counts_are_zero(file_metadata) which parses the footer created_by and returns true only for:
  • parquet-rs with version < 53.1.0
  • datafusion with version < 42.1.0
  • everything else (other writers, unparsable/missing created_by) -> false
• statistics_from_parquet_metadata now passes that flag to StatisticsConverter::with_missing_null_counts_as_zero, so file statistics (used by aggregates, constant pruning and sort pushdown) treat missing counts as Exact(0) only for legacy writers and stay Precision::Absent/Inexact otherwise.
• prune_by_statistics_with_metadata passes the footer FileMetaData down to the row-group pruner so both the forward pruner and the fully-matched (limit-pruning) pass use the same per-writer interpretation. The full prune_by_statistics (no-footer, test-only) path keeps its original true default for forward pruning but uses false for the fully-matched pass to stay sound.
• The runtime RowGroupPruner in push_decoder.rs derives its flag from the footer created_by instead of hard-coding true.
• Added a 56.0.0 upgrade-guide note.

What is the testing strategy for this PR?

• New unit test missing_null_counts_are_zero in metadata.rs covering parquet-rs/datafusion version boundaries, non-matching writers, unparsable versions and absent created_by.
• New fixture-variant test missing_null_counts_are_zero_only_for_writers_that_omitted_them in statistics_order_tests.rs. It builds a 3-row-group file whose footer is rewritten with created_by set to an old parquet-rs writer, an old datafusion writer, and several other writers, with null_count stripped exactly as parquet-rs < 53.1.0 wrote it. It asserts:
  • file statistics are Exact(3)/Exact(0) for legacy writers, Inexact(3)/Absent for others
  • static row-group pruning skips groups 0 and 2 for IS NULL only for legacy writers (returns [1]), keeping all groups for other writers
  • the runtime pruner in push_decoder.rs matches the static pruning
  • 3 matching rows are still found through every pruning level
• cargo test -p datafusion-datasource-parquet --lib: 257 passed, 0 failed
• cargo check/cargo clippy --all-targets --no-deps -- -D warnings: clean
• Note: the datafusion-core-parquet integration suites could not be built on this Windows machine due to a pre-existing rustc 1.97 STATUS_STACK_BUFFER_OVERRUN crash while compiling the windows crate (also affects the unmodified base commit).

Are there any user-facing changes?

Yes, an upgrade-guide note was added to docs/source/library-user-guide/upgrading/56.0.0.md. Users of files written by old parquet-rs/DataFusion writers gain back exact statistics, row-group pruning and sort pushdown; files from other writers that omit null counts are now handled conservatively instead of returning wrong results.