feat: add DNS server to guest boot args

[slash-init]

Description

This adds DNS server handling for guests that support configuring DNS through boot/runtime arguments.

The DNS server is read from the OCI /etc/resolv.conf mount and passed through NetDevParams to the guest-specific network configuration.

Currently this adds DNS handling for:

• Mirage
• Hermit
• Mewz

Tests have also been added for the new DNS handling.

Rumprun is left out for now based on the discussion in #1039, since it does not appear to support configuring DNS through boot arguments.

Related issues

• Fixes Read and set the DNS server for every guest #1039

How was this tested?

• Added unit tests for Mirage and Mewz DNS handling.
• Ran the relevant Go tests locally.
• The OCI /etc/resolv.conf -> NetDevParams.DNSServer path still needs end-to-end verification.

LLM usage

GPT 5.6 Luna was used to assist with implementation. All generated code was reviewed.

Checklist

• I have read the [contribution guide](https://urunc.io/developer-guide/contribute/).
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the [llm policy](https://urunc.io/developer-guide/llm-policy/).

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[slash-init]

hey @cmainas , just following up on this PR since it's been a couple weeks.
the current changes implement DNS boot-arg handling for Mirage, Hermit and Mewz, with tests for the new behavior. Rumprun is left out based on the discussion in the issue.
happy to address any feedback or changes needed.

[cmainas]

Hello @slash-init ,

please sign your commits.

[cmainas]

The commit is still unsigned.

[slash-init]

@cmainas thanks for pointing that out
i’ve added the DCO sign-off to the commit and force-pushed the updated commit.

[cmainas]

Thank you @slash-init for the updates. I have added some comments. Also, please do not forget to add yourself in https://github.com/urunc-dev/urunc/blob/main/.github/contributors.yaml

[slash-init]

@cmainas thanks!
addressed the review feedback and added myself to the contributors list.