GH-47417: [C++] Fix JSON parser losing nulls in a null-typed list

[advitrocks9]

Rationale for this change

{"a": [null, 1]} parses to [[1]]. Nothing raises, and validate(full=True) is the only thing
that notices:

Offset invariant failure: offset for slot 1 out of bounds: 2 > 1

Debug builds abort on a DCHECK instead, at parser.cc:540 if the second element is null and
parser.cc:1151 if it is typed.

What changes are included in this PR?

RawBuilderSet::AppendNull takes the child builder by value. Builders of Kind::kNull have no
arena, their null count is packed into BuilderPtr::index, so the increment updates the parent
but is lost to the caller. The next null then re-increments the same stale value, which leaves an
all-null list as NullArray(1) and a promoted list with its leading nulls missing.

It now takes a BuilderPtr*, matching MakeBuilder in the same class. The regression came in
with b7054c21aab and has shipped since 0.14.0.

Are these changes tested?

BlockParser.NullsInList covers both arms. arrow-json-test passes 261/261 in Debug and
Release. Reverting parser.cc with the test in place aborts in Debug and fails both columns in
Release.

Are there any user-facing changes?

Those inputs parse correctly now. This is not inference-only: an explicit list(null()) schema
reaches the same arm under both Error and Ignore.

This PR contains a "Critical Fix". Well-formed JSON silently loses list elements in release
builds and aborts on a DCHECK in debug builds.

• GitHub Issue: [C++] Assertion fires when trying to parse json #47417

[github-actions]

⚠️ GitHub issue #47417 has been automatically assigned in GitHub to PR creator.

[Copilot]

Pull request overview

Fixes a long-standing JSON parsing bug in the C++ BlockParser where null elements could be silently dropped (or trigger DCHECKs in debug) when appending nulls to a null-typed list, by ensuring the inline null-count state in BuilderPtr is updated in-place.

Changes:

• Change RawBuilderSet::AppendNull to take a BuilderPtr* so increments to the inline null-count for Kind::kNull persist to the caller.
• Update all AppendNull call sites to pass builder pointers accordingly (including nested object-field null propagation).
• Add a regression test covering nulls inside lists (null-only and promoted-from-null cases).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cpp/src/arrow/json/parser.cc	Fixes null appends by mutating the BuilderPtr in-place, preventing lost null counts in null-typed list builders.
cpp/src/arrow/json/parser_test.cc	Adds a regression test to ensure nulls inside lists are preserved and don’t break list offsets.

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