[Python][C++] utf8_normalize ignores its `form` option: NFC and NFKC output is decomposed

[singhpratech]

Describe the bug

pyarrow.compute.utf8_normalize returns decomposed text for every form. With form="NFC" the
output is the NFD form (base letter plus combining mark), whether the input was precomposed or
decomposed. unicodedata.normalize from the standard library gives the expected result.

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

import pyarrow as pa, pyarrow.compute as pc, unicodedata

s = "héllo"                      # precomposed e-acute, U+00E9
a = pa.array([s], pa.string())
for form in ("NFC", "NFD", "NFKC", "NFKD"):
    out = pc.utf8_normalize(a, form=form).to_pylist()[0]
    exp = unicodedata.normalize(form, s)
    print(form, [hex(ord(c)) for c in out], "expected", [hex(ord(c)) for c in exp])

Output:

NFC  ['0x68', '0x65', '0x301', '0x6c', '0x6c', '0x6f'] expected ['0x68', '0xe9', '0x6c', '0x6c', '0x6f']
NFD  ['0x68', '0x65', '0x301', '0x6c', '0x6c', '0x6f'] expected ['0x68', '0x65', '0x301', '0x6c', '0x6c', '0x6f']
NFKC ['0x68', '0x65', '0x301', '0x6c', '0x6c', '0x6f'] expected ['0x68', '0xe9', '0x6c', '0x6c', '0x6f']
NFKD ['0x68', '0x65', '0x301', '0x6c', '0x6c', '0x6f'] expected ['0x68', '0x65', '0x301', '0x6c', '0x6c', '0x6f']

The same happens when the input is already decomposed ("héllo"): NFC and NFKC return it
unchanged instead of composing to U+00E9.

Expected behavior

form="NFC" and form="NFKC" compose, as unicodedata.normalize does; NFD and NFKD are correct
today.

Component(s)

Python, C++

[singhpratech]

Reading the kernel on main (cpp/src/arrow/compute/kernels/scalar_string_utf8.cc, Utf8NormalizeBase):
MakeDecomposeOptions returns UTF8PROC_COMPOSE for NFC and NFKC, but the only utf8proc call the
kernel makes is utf8proc_decompose, which decomposes regardless of that flag; composition happens
in utf8proc_reencode, which is never called. So NFC and NFKC come out as NFD and NFKD. Calling
utf8proc_reencode on the scratch buffer with the same options after the decompose (as
utf8proc_map does internally) would give the composed forms.