examples: add idna-add-post-recheck

[astrogilda]

Add an examples/ patch that rechecks the output of a UTS-46 mapping idna.*.ToASCII call for an IP literal.

UTS-46 NFKC mapping folds some Unicode digits to their ASCII equivalents. A host that does not parse as an IP literal before mapping can parse as one after, bypassing pre-mapping checks. The patch inserts:

if _, ipErr := netip.ParseAddr(strings.TrimRight(out, ".")); ipErr == nil {
    return "", errIDNAIPLiteralSmuggle
}

TrimRight handles multiple trailing dots. The trim applies only to the ParseAddr argument, leaving the ToASCII result unchanged.

This is the gopatch equivalent of Semgrep rule semgrep/semgrep-rules#3841 and CodeQL query github/codeql#21784.

Notes:

• Matches the out, err := ...ToASCII(...) assignment form. Direct return ...ToASCII(x) is not matched.
• Assumes a (string, error) return signature.
• The error sentinel is assumed to be a package-level var. Since gopatch lacks an add-if-absent primitive for top-level declarations, the sentinel must be added separately.
• Punycode ToASCII, StrictNoIPLiteral(), and url.Parse callers are out of scope.

I will sign the CLA.

[CLAassistant]

All committers have signed the CLA.

[astrogilda]

Open since 25 June with the CLA green, no reviewer requested and no comment.

Before I spend more on it: does this project accept contributed examples at all? If not, say so and I will close it. The example rewrites a call site to add the post-IDNA IP-literal recheck.