[Sonike]

Summary

• convert std::io::Errors wrapped by anyhow into Python OSErrors at the UDP and WireGuard server boundaries
• preserve the raw OS errno so mitmproxy can recognize EADDRINUSE and show its existing actionable port suggestion
• add a regression test using a real address-in-use error and document the change

Fixes mitmproxy/mitmproxy#7650.

Test plan

• cargo fmt --all --check
• cargo clippy -p mitmproxy_rs --all-targets -- -D warnings
• cargo test -p mitmproxy_rs

This change was prepared with Codex assistance and verified locally.

[mhils]

Thank you for the great PR! 😃