fix(network): allow inspecting pseudo networks

[immanuwell]

refs #3005

nerdctl network ls shows host and none, but nerdctl network inspect host and nerdctl network inspect none still fail. the inspect path asks to allow pseudo networks, but lookup only checked on-disk CNI configs, so these names fell through. kinda awkward.

this returns a small synthetic config for pseudo networks in that lookup path, so inspect can render them fine.

repro:

1. run nerdctl network ls
2. see host and none
3. run nerdctl network inspect host
4. before: fails with no network found matching: host
5. after: returns JSON with "Name": "host"

checks:

• go test ./pkg/netutil -count=1
• go test ./pkg/inspecttypes/dockercompat -count=1
• go test ./cmd/nerdctl/network -run ^ -count=1

[AkihiroSuda]

 FAIL - does not have a valid DCO

https://github.com/containerd/nerdctl/actions/runs/27420363683/job/81044018040?pr=4953

[AkihiroSuda]

Isn't this struct already defined in somewhere else?

[immanuwell]

good point
I switched newPseudoNetworkConfig to reuse the existing cniNetworkConfig, and made nerdctlID / nerdctlLabels omitempty so the synthetic pseudo-network config stays minimal
PTAL

[Copilot]

Pull request overview

This PR fixes nerdctl network inspect for pseudo networks (host and none) by synthesizing a minimal, parseable CNI config when those names are requested, allowing the existing inspect rendering path to work consistently.

Changes:

• Extend CNIEnv.ListNetworksMatch() to return a synthetic NetworkConfig for pseudo networks when allowed.
• Add a small pseudoNetworkPlugin type and a helper to build the synthetic config.
• Enable/expand test coverage so network inspect host|none now passes.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
pkg/netutil/netutil.go	Adds pseudo-network lookup handling and synthetic config generation; tweaks JSON tags for optional fields.
pkg/netutil/netutil_test.go	Adds a unit test ensuring pseudo networks are included in match results.
pkg/netutil/cni_plugin.go	Introduces a minimal plugin type used for pseudo network config serialization.
cmd/nerdctl/network/network_inspect_test.go	Removes the “NeedsFixing” gating so inspect host/none is exercised normally.

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[AkihiroSuda]

test / in-host / docker failed 7 times
https://github.com/containerd/nerdctl/actions/runs/27458993998/job/81517770971?pr=4953

2026-06-15T20:35:11.1401312Z === Failing tests ===
2026-06-15T20:35:11.1401617Z TestRunSeccompCapSysPtrace
2026-06-15T20:35:11.1401881Z TestUpdateRestartPolicy
2026-06-15T20:35:11.1402103Z =====================