chore: prepare v1.2.56 release

[X-PG13]

Summary

• bump package and runtime versions to 1.2.56
• add CHANGELOG and release notes for the v1.2.56 maintenance release
• update English and Simplified Chinese release notes indexes

Validation

• ./.venv-dev/bin/python -m unittest tests.test_docs_links tests.test_release_metadata -v
• git diff --check
• make check PYTHON=./.venv-dev/bin/python

Release milestone: v1.2.56