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Let client ca post start hook use GET to check if the system namespace exists
This is a direct fix for #56761.
The original code tried to create a namespace unconditionally, it caused apiserver to fail to reboot if a webhook blocked namespaces creation.
In the long term, we should make the apiserver return 409 instead of 5xx in the case of conflicting POST, even if a webhook fails the POST.
65c0d55bba