🐛Fix process Stop timeout error handling

[mattsu2020]

What changed

• Preserve the Stop timeout error when SIGKILL fails after StopTimeout expires.
• Include the SIGKILL failure as supplementary error context with errors.Join.
• Add a deterministic regression test for the timeout-plus-kill-failure path.

Why

On Darwin, SIGKILL can fail with EPERM if the process is already terminating after the timeout. That failure previously replaced the primary timeout error, making callers and tests see unable to kill process ... instead of timeout waiting for process ... to stop.

Fixes #3522.

Validation

go test ./pkg/internal/testing/process/
go test ./pkg/internal/testing/process/ -run TestStopReturnsTimeoutWhenKillAfterTimeoutFails -count=100

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mattsu2020
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[k8s-ci-robot]

Hi @mattsu2020. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[mattsu2020]

Hi @joelanford @troy0820 @vincepri,

This PR is ready for review. The PR title verifier is now passing, but golangci-lint is still waiting for maintainer approval because the PR has the needs-ok-to-test label.

Could one of you please take a look and run /ok-to-test if the patch looks reasonable?

Thanks!

[sbueringer]

/ok-to-test

[sbueringer]

Needs rebase after #3519 is merged