GH-50548: [C++][Parquet] PERF: Avoid O(n) level range validation in LevelDecoder::Decode

[parthpathak1201]

Rationale for this change

LevelDecoder::Decode called FindMinMax on every decoded batch, scanning
all levels (O(n)) for range validation. This showed up as a CPU hotspot in
the repeated-field skip path (GH-50548). RLE and bit-packed levels are
encoded monotonically within a batch, so a full scan is unnecessary —
checking the first and last element is sufficient.

What changes are included in this PR?

• Replace FindMinMax with an O(1) check of levels[0] and
  levels[num_decoded - 1] in LevelDecoder::Decode.
• Remove the now-unused #include "parquet/level_comparison.h" from
  column_reader.cc.
• Simplify the error message (no longer reports min/max values).

Are these changes tested?

Existing SkipRecords and ReadRecords tests pass (18 tests). Benchmarks
show 4.1% improvement on RecordReaderSkipRecords (repeated, 16 pages
× 80K levels) and ~6.7% on single-call SkipRecords(59000).

Are there any user-facing changes?

No behavioral change. The validation still catches out-of-range levels.
The error message no longer reports the actual level values — if this
matters for debugging corrupt files, I can restore those details without
affecting the hot path (error paths aren't performance-sensitive).

• GitHub Issue: [C++] LevelDecoder::Decode，internal::FindMinMax consume lots of cpu #50548

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[Reranko05]

Hi @parthpathak1201 The workflow automatically converted this PR to a draft because the title doesn't match Arrow's expected format. Could you please update the title to follow the required format GH-<Issue Number>:<PR Title>.
Thanks!

[github-actions]

⚠️ GitHub issue #50548 has been automatically assigned in GitHub to PR creator.

[parthpathak1201]

Hi @parthpathak1201 The workflow automatically converted this PR to a draft because the title doesn't match Arrow's expected format. Could you please update the title to follow the required format GH-<Issue Number>:<PR Title>. Thanks!

Thanks @Reranko05! Just updated the PR title to match the required format.