Preserve pod DNS options when translating ClusterFirst DNS

[Kunalbehbud]

What issue type does this pull request address? (keep at least one, remove the others)
/kind bugfix

What does this pull request do? Which issues does it resolve? (use resolves #<issue_number> if possible)
resolves #1120

When a pod uses ClusterFirst DNS, translateDNSClusterFirstConfig builds a DNS config (nameserver + search domains + an injected ndots=5) and then merges the pod's own config back in — but only its Nameservers and Searches:

existingDNSConfig := pPod.Spec.DNSConfig
if existingDNSConfig != nil {
    dnsConfig.Nameservers = deleteDuplicates(append(dnsConfig.Nameservers, existingDNSConfig.Nameservers...))
    dnsConfig.Searches = deleteDuplicates(append(dnsConfig.Searches, existingDNSConfig.Searches...))
}

Options was never merged, so anything a pod set in dnsConfig.options (e.g. single-request-reopen, or a custom ndots) was silently dropped — exactly what the issue reports.

This merges the options too, via a small mergeDNSOptions helper. An option set on the pod takes precedence over the injected default with the same name, so an explicit ndots: "2" replaces the injected ndots=5 rather than colliding with it; options the pod sets that aren't in the defaults are simply appended.

Please provide a short message that should be published in the vcluster release notes
Fixed an issue where vcluster dropped a pod's dnsConfig.options when translating ClusterFirst DNS.

What else do we need to know?

translateDNSClusterFirstConfig is a pure function over corev1.Pod, so this is covered by a table-driven unit test (TestTranslateDNSClusterFirstConfigOptions) with no cluster required: default kept when the pod sets nothing, pod options preserved alongside the default, and pod ndots overriding the injected one. The test fails on main and passes with this change.

---

Note

Low Risk
Small, localized pod DNS translation fix with table-driven unit tests and no auth or data-path changes.

Overview
Fixes a bug where virtual pods with ClusterFirst DNS lost dnsConfig.options on the host pod. translateDNSClusterFirstConfig already merged nameservers and search domains from the pod spec but ignored options, so settings like single-request-reopen or a custom ndots never reached the physical pod.

The merge path now calls mergeDNSOptions, which keeps injected defaults (including ndots=5) unless the pod defines the same option name—in which case the pod value wins. Other pod-only options are kept alongside the defaults.

TestTranslateDNSClusterFirstConfigOptions covers no existing config, extra pod options, and pod ndots overriding the default.

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