fix(network): use exact regex pattern in getTapIndex

[Norway-02]

/kind bug

Rationale

In Kubernetes and containerized environments, network interface names created by host systems or CNI plugins often contain "tap" as a substring (e.g. bootstrap0, vtap0, cni-tap0, stape0).

Currently, getTapIndex() in pkg/network/network.go checks:

if strings.Contains(iface.Name, "tap") {
    tapCount++
}

When DynamicNetwork.NetworkSetup() runs during container creation, if tapIndex > 0, urunc aborts container launch with:
unsupported operation: can't spawn multiple unikernels in the same network namespace

This causes urunc to falsely believe a unikernel is already running and refuse to spawn the first container whenever any interface with "tap" in its name exists in the network namespace.

This PR updates getTapIndex() to use the regex pattern ^tap\d+(_urunc)?$, matching urunc's own TAP device naming convention while ignoring unrelated host/CNI interfaces.

Fixes #938

Reviewer Notes

• Added unit tests TestGetTapIndex and TestTapDeviceRegexMatching in pkg/network/network_test.go.
• Tagging maintainers @cmainas @OdysseasKalaitsidis for review.

LLM usage

none

fix(network): prevent false positive TAP interface matching in getTapIndex()

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