chore: bump version to 0.23.2 + CHANGELOG entry
Satisfies the check-version CI gate (every PR off a released main must open a new
version) and the changelog-enforcer (this touches unstructured/__version__.py).
Documents the partition-benchmark CI stabilization in this PR.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
