fix(network): distinguish missing container iface from setup failures

[Vi-shub]

Description

SetupNet previously logged and ignored all NetworkSetup errors, so real failures (TAP creation, TC rules, permissions) could leave a container running without networking.
This change follows the guidance on #417:

• Add network.ErrNoContainerNetwork for the "no suitable interface in the netns" case (ctr / --network none).
• discoverContainerIface returns that sentinel.
• SetupNet continues only when errors.Is(err, ErrNoContainerNetwork); all other errors are returned to the caller.
• Align static network discovery wrapping with the dynamic path so the sentinel stays detectable via %w.

Related issues

• Fixes SetupNet silently ignores network setup failures, causing containers to run without networking #417

How was this tested?

• make test_network (includes TestErrNoContainerNetworkIsDetectableWhenWrapped)
  • sudo nerdctl run --rm --network none --runtime io.containerd.urunc.v2 harbor.nbfc.io/nubificus/urunc/hello-qemu-unikraft:latest → Hello world
  • sudo nerdctl run -d --name urunc-nginx --runtime io.containerd.urunc.v2 harbor.nbfc.io/nubificus/urunc/nginx-qemu-unikraft-initrd:latest → curl to container IP succeeded

LLM usage

Claude assisted with drafting the PR creation; I reviewed and verified the logic against the discussion on #417 and tested locally as above.

Checklist

• I have read the contribution guide.
• 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.

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[Vi-shub]

Testing results

[Vi-shub]

Hi @cmainas ,
This addresses #417 as discussed. Please take a look when you have a chance. Thankyou for your time!