Validate IPv6 prefix field in VPC create form

[david-crespo]

Closes #3339

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Project	Deployment	Actions	Updated (UTC)
console	Ready	Preview	Aug 21, 2026 5:33pm

[david-crespo]

Vpc in the name is deliberate — there are VPC-specific rules here.

[david-crespo]

Checked against the Rust logic with the following cargo script:

#!/usr/bin/env -S cargo +nightly -Zscript
---
[dependencies]
oxnet = "=0.1.6"
---

use oxnet::Ipv6Net;
use std::str::FromStr;

fn backend_accepts(value: &str) -> bool {
    Ipv6Net::from_str(value)
        .is_ok_and(|prefix| prefix.is_unique_local() && prefix.width() == 48)
}

fn main() {
    let cases = [
        ("fd00::/48", true),
        ("fd2d:4569:88b2::/48", true),
        ("fd00::1/48", true),
        ("fc00::/48", true),
        ("fdff:ffff:ffff:ffff:ffff:ffff:ffff:ffff/48", true),
        ("nonsense", false),
        ("fd00::", false),
        ("10.0.0.0/8", false),
        ("::/48", false),
        ("fbff:ffff:ffff:ffff:ffff:ffff:ffff:ffff/48", false),
        ("2001:db8::/48", false),
        ("fe00::/48", false),
        ("fd00::/64", false),
        ("fd00::/40", false),
        ("fd00::/129", false),
    ];

    let mismatches: Vec<_> = cases
        .into_iter()
        .filter(|(value, expected)| backend_accepts(value) != *expected)
        .collect();

    if mismatches.is_empty() {
        println!("all {} cases match oxnet 0.1.6", cases.len());
    } else {
        for (value, expected) in mismatches {
            eprintln!("{value}: expected {expected}, got {}", backend_accepts(value));
        }
        std::process::exit(1);
    }
}