fix(dns): treat empty DNS response as an error

[aivus]

Summary

In this pull request, the following changes are made:

• DnsMonitorType.dnsResolve() now throws a descriptive error when the DNS resolver returns an empty array, instead of passing it downstream.

Background: when a hostname exists but has no records of the requested type (e.g. a host that only has a CNAME/A record queried for AAAA), Node's Resolver.resolve() resolves with an empty array rather than throwing ENODATA (documented behaviour: nodejs/node#21795). Previously this surfaced as a DOWN monitor with a confusing empty Records: message. The monitor now reports No AAAA records found for <hostname> (or the relevant record type/hostname).

The Array.isArray guard leaves object-returning types like SOA unaffected, and PTR (reverse lookup) is handled before this check.

• Resolves Better handling of empty DNS response #4979

Please follow this checklist to avoid unnecessary back and forth (click to expand)

• ⚠️ If there are Breaking change (a fix or feature that alters existing functionality in a way that could cause issues) I have called them out
• 🧠 I have disclosed any use of LLMs/AI in this contribution and reviewed all generated content. I understand that I am responsible for and able to explain every line of code I submit.
• 🔍 Any UI changes adhere to visual style of this project.
• 🛠️ I have self-reviewed and self-tested my code to ensure it works as expected.
• 📝 I have commented my code, especially in hard-to-understand areas (e.g., using JSDoc for methods).
• 🤖 I added or updated automated tests where appropriate.
• 📄 Documentation updates are included (if applicable).
• 🧰 Dependency updates are listed and explained.
• ⚠️ CI passes and is green.

Testing

Added test/backend-test/monitors/test-dns.js covering both the empty-response (throws) and non-empty-response (returns records) cases, using node:test mocks so it does not depend on the network:

✔ dnsResolve() throws when the DNS response is an empty array
✔ dnsResolve() returns records when the DNS response is not empty

🤖 Generated with Claude Code

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Re-creation of #7598 due to the fact that PR is stuck on GH side

[github-actions]

Thanks for the PR! If anyone would like to help with testing, run: npx kuma-pr aivus:fix/empty-dns-response-4979 (requires Node.js and Docker)