fix: preserve prefer_udp in LocalDNS Corefile

[Saewon Kwak (saewoni)]

Change summary

Preserves CoreDNS prefer_udp behavior when LocalDNS overrides specify Protocol: PreferUDP.

• Updates the aks-node-controller LocalDNS Corefile template to render prefer_udp in forward blocks when the override protocol is PreferUDP.
• Updates the legacy pkg/agent LocalDNS Corefile renderer with the same prefer_udp handling so scriptless and non-scriptless render paths stay consistent.
• Keeps existing force_tcp behavior intact for Protocol: ForceTCP.
• Updates LocalDNS Corefile golden/assertion coverage so PreferUDP is emitted for both render paths.

This fixes a mismatch where LocalDNS override data could carry Protocol: PreferUDP, but the generated Corefile did not preserve that setting.

Testing

• GOPROXY=https://proxy.golang.org,direct go test ./pkg/agent
• cd aks-node-controller && GOPROXY=https://proxy.golang.org,direct go test ./...

[Copilot]

Pull request overview

This PR ensures the LocalDNS CoreDNS “forward” plugin option prefer_udp is preserved when LocalDNS overrides specify Protocol: PreferUDP, across both the non-scriptless (pkg/agent) and scriptless (aks-node-controller) Corefile render paths.

Changes:

• Update pkg/agent LocalDNS Corefile template to emit prefer_udp when override.Protocol == "PreferUDP".
• Update aks-node-controller LocalDNS template to emit prefer_udp for the same override protocol.
• Refresh golden/expected Corefile outputs and assertions in unit tests to include prefer_udp.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
pkg/agent/baker.go	Emit prefer_udp in LocalDNS Corefile rendering when overrides request PreferUDP.
pkg/agent/baker_test.go	Update expected Corefile strings/assertions so PreferUDP output includes prefer_udp.
aks-node-controller/parser/templates/localdns.toml.gtpl	Emit prefer_udp in the scriptless LocalDNS Corefile template for PreferUDP overrides.
aks-node-controller/parser/helper_test.go	Update expected Corefile strings to include prefer_udp for PreferUDP test cases.

[github-actions]

Windows Unit Test Results

  3 files   12 suites   1m 4s ⏱️
389 tests 389 ✅ 0 💤 0 ❌
392 runs  392 ✅ 0 💤 0 ❌

Results for commit aefb861.