[siddiqueirshad]

Rationale for this change

Fixes #55295. Currently, Dataset.head, Scanner.head, and Fragment.head accept negative num_rows (e.g. head(-1)). AsyncScanner::Head passes the negative count directly into RecordBatch::Slice(0, num_rows), producing invalid record batches with negative lengths that cause Table.validate() to fail with Array length is negative and len() in Python to fail with SystemError.

What changes are included in this PR?

• In AsyncScanner::Head, check if (num_rows < 0) and return Status::Invalid("num_rows must be non-negative").
• In C++ cpp/src/arrow/dataset/scanner_test.cc, add ASSERT_RAISES(Invalid, scanner->Head(-1)) to TestScanner.Head.
• In Python python/pyarrow/tests/test_dataset.py, test that negative rows raise ValueError (as ArrowInvalid subclasses ValueError) for both dataset_reader.head(dataset, -1) and fragment.head(-1).

Are these changes tested?

Yes, added C++ unit test in scanner_test.cc and Python tests in test_dataset.py.

Are there any user-facing changes?

Yes, calling head() with a negative num_rows now returns an ArrowInvalid error instead of returning a malformed Table with negative row count.

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PR code and description written by:

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Reviewed before submission by:

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• GitHub Issue: [Python] Dataset.head(-1) returns a Table with negative num_rows #55295

[github-actions]

⚠️ GitHub issue #55295 has been automatically assigned in GitHub to PR creator.

[github-actions]

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