[C++] winsorize on a sliced array ignores the offset: wrong rows come out null

[singhpratech]

Describe the bug

pyarrow.compute.winsorize on an array with a non-zero offset returns nulls at positions that hold
values and values at positions that are null, even with limits of 0.0 and 1.0, which should return
the input unchanged. The same values in an array with offset 0 come back unchanged.

pyarrow 25.0.1, Python 3.13.9, macOS 26.6 (arm64), wheel from PyPI.

import pyarrow as pa, pyarrow.compute as pc

flat = pa.array([1.0, 2.0, None, 4.0, None, 6.0, 7.0, 8.0], pa.float64())
sliced = flat.slice(2, 5)                       # [None, 4.0, None, 6.0, 7.0]
copy = pa.array(sliced.to_pylist(), pa.float64())

print("sliced", pc.winsorize(sliced, lower_limit=0.0, upper_limit=1.0).to_pylist())
print("copy  ", pc.winsorize(copy, lower_limit=0.0, upper_limit=1.0).to_pylist())

Output:

sliced [0.0, 4.0, None, 6.0, None]
copy   [None, 4.0, None, 6.0, 7.0]

The null pattern of the sliced result, [valid, valid, null, valid, null], is the parent's validity
bitmap read from bit 0 (flat[0..5] = valid, valid, null, valid, null) rather than from bit 2.

Expected behavior

winsorize(sliced, 0.0, 1.0) returns [None, 4.0, None, 6.0, 7.0], the same as for the copy.

Component(s)

C++, Python