Fix flaky hnswlib stubtest allowlist

[nightcityblade]

Fixes #16100

The daily Linux job did not report the hnswlib.Index metaclass mismatch and therefore rejected the existing allowlist entry as unused, while the PR's fresh Linux job reproduced the mismatch. Keep the exception cross-platform, but make the regular-expression entry optional so stubtest accepts both runtime variants.

Tests:

• PYTHONPATH=lib python3 tests/stubtest_third_party.py hnswlib -v
• PYTHONPATH=lib python3 tests/check_typeshed_structure.py
• uvx pre-commit run --files stubs/hnswlib/@tests/stubtest_allowlist.txt

[nightcityblade]

Addressed the Linux stubtest failure in 8e76d4e.

The PR job reproduced the pybind11_type mismatch on Linux even though the daily Linux run did not, so a Darwin-only allowlist was insufficient. stubs/hnswlib/@tests/stubtest_allowlist.txt now uses the optional regex (hnswlib.Index)?, which accepts either runtime result without an unused-entry failure.

Validation:

• PYTHONPATH=lib python3 tests/stubtest_third_party.py hnswlib -v — passed
• PYTHONPATH=lib python3 tests/check_typeshed_structure.py — passed
• pre-commit on the allowlist — passed