fix: always append IPv6 bootstrap DNS servers

[Itsusinn]

Problem

On an IPv6-only uplink (e.g. 464XLAT cellular, where the network interface has no IPv4 address), the app fails to resolve any domain — every connection logs dns error: no DNS clients available for query.

Trace from a real device log (rmnet_data5 has v4: None, only a global IPv6 address):

1. DNS bootstrap (default_nameserver) runs with IPv4-only servers (114.114.114.114, 8.8.8.8) → both unreachable → every bootstrap query times out.
2. Because bootstrap failed, the DoT main nameservers (dns.alidns.com:853, dot.pub:853) can't be resolved → initializing DNS client ... with error dns error: request timed out.
3. all components initialized, but with zero usable DNS clients → no DNS clients available for query on every subsequent connection.

Why #24 didn't cover this

#24 added IPv6 bootstrap servers, but only inside the if config.dns.default_nameserver.is_empty() branch. This device's profile supplies its own default-nameserver (the IPv4-only 114.114.114.114 value — not present anywhere in our code — confirms it), so it took the else branch and the IPv6 fallback was silently dropped.

Change

• Always append the IPv6 bootstrap servers (AliDNS 2400:3200::1 / 2400:3200:baba::1, Google 2001:4860:4860::8888) to default_nameserver, whether the list came from the built-in defaults or from the profile. A dedup check (url::Host derives PartialEq) avoids duplicates.
• Factored the repeated NameServer { … } literals into a small udp_bootstrap closure.

Reviewer notes

• Type-correct: the dedup compare compiles because Host is url::Host (derives PartialEq); the closure borrow in .any() ends before the push move.
• Not yet verified with an Android NDK build / on-device test on an IPv6-only network — would appreciate a check before release. Expected behavior: bootstrap also tries the IPv6 clients and at least dns.alidns.com:853 initializes successfully.
• Out of scope: a profile supplying its own IPv4-only nameserver (the main resolvers, not bootstrap) would still fail on IPv6-only. Not the case here (the main resolvers are DoT domains, which resolve fine once bootstrap works).

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