ProxyStrategies: code cleanup and improve test coverage.

[jkh52]

• Validate the "no strategies" case (such a server would be non-functional).
• Improves pkg/server/backend_manager.go code coverage from 72.6% to 86.3%.

[jkh52]

Test flake:

--- FAIL: TestProxyDial_RequestCancelled_Concurrent_GRPC (61.92s)
    proxy_test.go:399: Timed out waiting for tunnel to close
    proxy_test.go:426: Agent connections leaked: 
        
        Diff:
        --- metric output does not match expectation; want
        +++ got:
        @@ -2,3 +2,3 @@
         # TYPE konnectivity_network_proxy_agent_open_endpoint_connections gauge
        -konnectivity_network_proxy_agent_open_endpoint_connections 0
        +konnectivity_network_proxy_agent_open_endpoint_connections 1

[jkh52]

/retest

[jkh52]

/retest

[jkh52]

The linter is failing despite my attempt to fix it (see second commit here).

Note that this PR uses go1.21 feature 'slices.Contains', I think that's the trigger.

@tallclair or @liangyuanpeng - make lint passes on one of my dev machines, but fails checks here. Could the problem be that we need a newer kubekins image? Or, should we just pass it an explicit GO_VERSION >= 1.21?

EDIT: I side-stepped this and moved the lint update to #605

[jkh52]

Latest snapshot removes the "duplicate" validation per offline discussion with @cheftako.

/assign @cheftako

[jkh52]

Test flake for "e2e / e2e (dual, v1.28.7)":

Run # output_dir
ERROR: failed to create cluster: failed to list nodes: command "docker ps -a --filter label=io.x-k8s.kind.cluster=kind --format '{{.Names}}'" failed with error: exit status 1

Command Output: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

Stack Trace: 
sigs.k8s.io/kind/pkg/errors.WithStack
	sigs.k8s.io/kind/pkg/errors/errors.go:59
sigs.k8s.io/kind/pkg/exec.(*LocalCmd).Run
...
github.com/spf13/cobra.(*Command).execute
	github.com/spf13/cobra@v1.4.0/command.go:8[56](https://github.com/kubernetes-sigs/apiserver-network-proxy/actions/runs/8574515244/job/23501575878?pr=604#step:5:57)
github.com/spf13/cobra.(*Command).ExecuteC
	github.com/spf13/cobra@v1.4.0/command.go:974
github.com/spf13/cobra.(*Command).Execute
	github.com/spf13/cobra@v1.4.0/command.go:902
sigs.k8s.io/kind/cmd/kind/app.Run
	sigs.k8s.io/kind/cmd/kind/app/main.go:53
sigs.k8s.io/kind/cmd/kind/app.Main
	sigs.k8s.io/kind/cmd/kind/app/main.go:35
main.main
	sigs.k8s.io/kind/main.go:25
runtime.main
	runtime/proc.go:250
runtime.goexit
	runtime/asm_amd64.s:1[59](https://github.com/kubernetes-sigs/apiserver-network-proxy/actions/runs/8574515244/job/23501575878?pr=604#step:5:60)8
Error: Process completed with exit code 1.

/retest

[jkh52]

Lint error:

pkg/server/backend_manager_test.go:458:21: unusedresult: result of (sigs.k8s.io/apiserver-network-proxy/pkg/server.ProxyStrategy).String call not used (govet)
					tc.input.String()

[cheftako]

If we're going to panic we need to have a very clear error/log message first. Some clue about what ps is set to is pretty essential.

[jkh52]

Done.

[cheftako]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cheftako, jkh52

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