Fix JUnit XML escaping for supplementary Unicode

[Sean-Kenneth-Doherty]

Fixes #14483.

Summary

• Use an 8-digit Unicode escape for the XML supplementary-plane range in bin_xml_escape.
• Restore regression coverage for valid XML boundary code points, including U+10000, emoji, and U+10FFFF.
• Add the changelog entry and AUTHORS entry requested by the PR template.

The previous static regex used က0-ჿff, but Python \u escapes consume only four hex digits. That left supplementary-plane characters outside the valid XML character range and caused valid emoji/CJK extension characters to be escaped as #x....

Validation

• .venv/bin/python -m pytest testing/test_junitxml.py::test_invalid_xml_escape -q -> 1 passed
• .venv/bin/python -m pytest testing/test_junitxml.py -q -> 135 passed, 2 skipped
• uvx ruff@0.15.12 check src/_pytest/junitxml.py testing/test_junitxml.py
• uvx ruff@0.15.12 format --check src/_pytest/junitxml.py testing/test_junitxml.py
• uvx --from mypy==2.0.0 --with iniconfig>=1.1.0 --with attrs>=19.2.0 --with pluggy>=1.5.0 --with packaging --with tomli --with types-setuptools --with types-tabulate --with exceptiongroup>=1.0.0rc8 mypy --pretty --show-error-codes --no-warn-unused-ignores src/_pytest/junitxml.py testing/test_junitxml.py -> no issues found
• uvx --from codespell==2.4.2 codespell --toml=pyproject.toml AUTHORS changelog/14483.bugfix.rst src/_pytest/junitxml.py testing/test_junitxml.py
• git diff --check origin/main...HEAD

Checklist

• Add text like closes #XYZW to the PR description and/or commits.
• If AI agents were used, they are credited in Co-authored-by commit trailers.
• Create a new changelog file in the changelog directory.
• Add yourself to AUTHORS in alphabetical order.

[Sean-Kenneth-Doherty]

Local/CI validation refresh from my side:\n\n- GitHub Actions matrix is green, including package, check, Read the Docs, pre-commit.ci, codecov/patch, and Python 3.10-3.14/PyPy lanes.\n- .venv/bin/python -m pytest testing/test_junitxml.py::test_invalid_xml_escape -q -> 1 passed\n- .venv/bin/python -m pytest testing/test_junitxml.py -q -> 135 passed, 2 skipped\n- git diff --check origin/main...HEAD -> clean

[RonnyPfannschmidt]

closing as agentic abuse after ignoring the prior close