Fix ipv6 discovery

[gfukushima]

PR Description

Vibe coded fix for the external address selection "bug" that was preventing teku from obtaining the ipv6 from discovery.

Fixed Issue(s)

Part of #10305

---

Note

Medium Risk
Changes external address auto-selection to update IPv4 and IPv6 ENR fields independently, which can affect how nodes advertise reachable endpoints. Main risk is incorrect ENR updates (e.g., flapping or missing one family) impacting peer connectivity in dual-stack environments.

Overview
Fixes dual-stack external address auto-discovery by selecting the best IPv4 and IPv6 external addresses independently instead of letting one IP family dominate and prevent the other from being published.

DefaultExternalAddressSelector now runs separate vote-based selection for IPv4/IPv6 and updates the corresponding ENR field via a shared maybeUpdateAddress path.

Test utilities and coverage are extended to support IPv6 ENR fields: SimpleIdentitySchemaInterpreter can now create/read udp6 records and preserves the other family when updating an address, and new DefaultExternalAddressSelectorTest cases validate both families can be discovered and that IPv6 selection is independent of IPv4 vote volume.

^{Written by Cursor Bugbot for commit 7db7228. This will update automatically on new commits. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[gfukushima]

I have read the CLA Document and I hereby sign the CLA

[zilm13]

LGTM