GH-35830: [C++] Fix fixed-size list scalar hashing with non-zero offsets

[HyukjinKwon]

Rationale for this change

Fixed-size list scalars with equal values but different offsets were hashing to different values.

What changes are included in this PR?

• Added FIXED_SIZE_LIST case in ArrayHash to scale child offsets by list_size
• Added C++ test in scalar_test.cc for nested fixed-size lists
• Added Python test in test_scalars.py

Are these changes tested?

Unit tests were added, and also manually tested.

Are there any user-facing changes?

Yes.

import pyarrow as pa

inner = pa.list_(pa.int32(), 2)
outer = pa.list_(inner, 3)
g = pa.array([[[1, 2], [3, 4], [5, 6]], [[7, 8], [9, 10], [11, 12]]], type=outer)
h = pa.array([[[7, 8], [9, 10], [11, 12]]], type=outer)
# Comparing `[[7, 8], [9, 10], [11, 12]]`
print(g[1] == h[0])
print(hash(g[1]) == hash(h[0]))

Before:

True
False

After:

True
True

• GitHub Issue: [C++] Handle only relevant slices of child arrays when hashing scalars from ListArrays #35830

[github-actions]

⚠️ GitHub issue #35830 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #35830 has been automatically assigned in GitHub to PR creator.