implement net.cidr_is_valid builtin

[tjons]

@anakrish I've been working on implementing the net.cidr_is_valid builtin. It currently is complete for IPv4, and so I wanted to try to get some early feedback while I finish support for IPv6.

I'm still quite new to Rust, and so please feel free to give any pointers or suggestions you think I might be missing 🦀

[tjons]

@microsoft-github-policy-service agree

[anakrish]

Hey @tjons,
Thanks for the contribution. Overall looks good to me. I've provided some suggestions.

[tjons]

@anakrish thanks for the feedback! I'll update this and finish ipv6 this week. Do let me know your thoughts on #422 (comment), which will either dramatically simplify the implementation or take a bit longer :)

[tjons]

@anakrish sorry for the wait, addressed all the feedback! Let me know if anything else is needed for this builtin, I'll get started on the rest this week.

[anakrish]

@tjons Overall looks good to me. Some suggestions on propagating the errors.

[anakrish]

Hey @tjons, Overall looks good to me.

There are a bunch of format string related failures. https://github.com/microsoft/regorus/actions/runs/16734832282/job/47424635159?pr=422

Can you also add v0/netcidrisvalid and v1/netcidrisvalid to https://github.com/microsoft/regorus/blob/main/tests/opa.passing to ensure that tests from OPA testsuite are run?

[tjons]

@anakrish done! I promise future contributions will be smoother, thanks for bearing with me, sorry this has taken a while as I'm learning the ropes.

While enabling the builtins in the tests, I had to refactor the code to not use Err returns, because the OPA built-in doesn't propagate errors when an invalid CIDR is passed and it was causing unsafe variable references. See https://github.com/open-policy-agent/opa/blob/main/v1/topdown/cidr.go#L220.

[anakrish]

@tjons No worries. I truly glad that you are contributing.