Fix truncated string statistics handling

[Flyangz]

Problem
When string statistics are truncated (long strings), the reader ignored lower_bound/upper_bound and defaulted to empty strings. This caused valid row groups to be incorrectly skipped during predicate pushdown.

Fix

1. Prioritize Exact Min/Max: Use minimum/maximum first; fallback to lower_bound/upper_bound if missing.
2. Handle Empty Rows: Return None for statistics immediately if number_of_values is 0.

These changes match Java writer logic in https://github.com/apache/orc/blob/495c5b364aa7763bc36d08c2ca4c41e5db968d0b/java/core/src/java/org/apache/orc/impl/ColumnStatisticsImpl.java#L797-L816

Test
Added unit tests.

[WenyXu]

Hi @Flyangz , The CI is failed

[Copilot]

Pull request overview

Fixes predicate pushdown correctness when ORC string statistics are truncated by properly interpreting minimum/maximum vs lower_bound/upper_bound, and introduces explicit “exactness” indicators to avoid incorrect row-group pruning.

Changes:

• Update string statistics decoding to prefer exact minimum/maximum and otherwise use lower_bound/upper_bound, tracking whether bounds are exact.
• Extend row-group string predicate evaluation to incorporate exact-vs-bound semantics.
• Update CLI/stat formatting outputs and golden expected output to include IsExactMin/IsExactMax.

Reviewed changes

Copilot reviewed 2 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/bin/expected/stats.out	Updates golden output to include IsExactMin/IsExactMax lines for string stats.
src/statistics.rs	Decodes string stats using exact min/max when available, otherwise uses lower/upper bounds and records exactness flags; returns no type stats when number_of_values == 0.
src/row_group_filter.rs	Uses the new string bound/exactness fields in predicate evaluation and adds unit tests for string comparison logic.
src/bin/orc/stats.rs	Prints exactness flags for string statistics in the orc stats CLI output.
src/bin/orc/common.rs	Includes exactness flags in formatted stats output for CLI/common formatting.

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[WenyXu]

@Flyangz Thanks!