fix: honor cluster-domain in worker DNS search paths

[immanuwell]

small follow-up to #704 and #738.

--cluster-domain already updates the hostfile and discover_hosts.sh, but worker pods still get dnsConfig.searches hardcoded to svc.cluster.local.
That bites Intel MPI and MPICH, workers need to resolve the launcher hostname too.

Fix:

• use the configured cluster domain when building worker DNS search suffixes
• add a regression test for a custom-domain Intel MPI job

Repro:

1. Start the operator with --cluster-domain=kube.local
2. Create an Intel MPI or MPICH job
3. Inspect a worker pod with kubectl get pod <worker> -o yaml
4. Before this patch it has dnsConfig.searches: ["<job>.<ns>.svc.cluster.local"]
5. The launcher FQDN is ...svc.kube.local, so hostname lookup is off and the job can break

[tenzen-y]

Ah, this is actually bug for MPICH and Intel MPI in case of the custom domain.
Thank you!

/lgtm
/approve

[google-oss-prow]

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