Handle non-UTF-8 names in packed refs

[pick7]

Summary

• Read packed-refs with Python surrogateescape handling.
• Preserve undecodable ref-name bytes instead of failing during decoding.
• Add a repo.tags regression test using a non-UTF-8 tag name.

Fixes #2064.

Validation

• GIT_PYTHON_TEST_GIT_REPO_BASE=/tmp/gitpython-test-fixture.i7gtuO .venv/bin/python -m pytest -o addopts= test/test_refs.py
• .venv/bin/python -m ruff check git/refs/symbolic.py test/test_refs.py
• .venv/bin/python -m ruff format --check git/refs/symbolic.py test/test_refs.py
• .venv/bin/python -m mypy git/refs/symbolic.py

AI agent disclosure

This pull request was prepared and submitted by OpenAI Codex acting as an AI agent through the contributor account.