[client] Fix review issues from IPv6 overlay PR

[lixmal]

Describe your changes

Address CodeRabbit review comments from #5631.

• Anonymize AddressV6 in debug bundles, preserving the original prefix length
• Anonymize IPv6 ULA addresses in debug bundles (the 40-bit global ID is a network fingerprint, unlike RFC 1918 IPv4 which is reused everywhere)
• Fix data race on extraListeners in SSH server Stop() by snapshotting under mutex
• Warn and clear IPv6 on Windows when IPv6 MTU setup fails, matching Linux/macOS behavior
• Clear stale DNS settings from the opposite address family in NetworkManager configurator
• Check AddPeer errors in status test setup instead of discarding them

Issue ticket number and link

Follow-up to #5631

Stack

Checklist

• Is it a bug fix
• Is a typo/documentation fix
• Is a feature enhancement
• It is a refactor
• Created tests that fail without the change (if possible)

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Documentation

Select exactly one:

• I added/updated documentation for this change
• Documentation is not needed for this change (explain why)

Internal code fixes, no user-facing behavior change.

Docs PR URL (required if "docs added" is checked)

Paste the PR link from https://github.com/netbirdio/docs here:

https://github.com/netbirdio/docs/pull/__

[coderabbitai]

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title '[client] Fix review issues from IPv6 overlay PR' directly references addressing CodeRabbit review comments from a related PR and is specific enough for scanning history.
Description check	✅ Passed	The pull request description follows the required template structure with all essential sections completed.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-ipv6-overlay-coderabbit-issues

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud