Handle malformed RFC 2231 continuations in parse_options_header

[manunio]

Summary

• handle both ValueError (oversized RFC 2231 index) and TypeError (mixed continuation forms) from Message.get_params()
• add regression tests covering oversized index and mixed RFC 2231 continuation inputs to prevent fuzz-found crashes

Why

parse_options_header relied on stdlib email parsing that can raise different exception types for malformed parameter continuations. These changes keep existing behavior while making parser failure handling robust and preventing constant fuzzer crashes(improves its performance)

Validation

• uv run pytest -q
• uv run ruff check python_multipart/exceptions.py python_multipart/multipart.py tests/test_multipart.py

[Kludex]

Thanks @manunio 🙏

[manunio]

I’m returning the header as is to avoid regex magic for parser differences between python versions.