fix: validate webhook deployment script arguments

[immanuwell]

What this PR does / why we need it:

deploy-vsphere-csi-validation-webhook.sh had busted CLI error handling.
Unknown flags tried to run usage like a command, and --namespace with no value died on a bare shift, so the script faceplanted before it even touched the cluster. Kinda rough.

This patch adds a real usage() helper, returns 0 for --help, validates --namespace, and keeps the normal path the same.

Which issue this PR fixes: None

Testing done:

bash -n manifests/vanilla/deploy-vsphere-csi-validation-webhook.sh

Repro with the old script blob from HEAD:
tmp_old=$(mktemp) && git show HEAD:manifests/vanilla/deploy-vsphere-csi-validation-webhook.sh > "$tmp_old" && chmod +x "$tmp_old" && bash "$tmp_old" --bad-flag
old output: usage: command not found

tmp_old=$(mktemp) && git show HEAD:manifests/vanilla/deploy-vsphere-csi-validation-webhook.sh > "$tmp_old" && chmod +x "$tmp_old" && bash -x "$tmp_old" --namespace
old behavior: exits after a bare shift

Repro with this patch:
bash manifests/vanilla/deploy-vsphere-csi-validation-webhook.sh --bad-flag
now prints a clean unknown-option error and usage text

bash manifests/vanilla/deploy-vsphere-csi-validation-webhook.sh --namespace
now prints a clean missing-value error and usage text

bash manifests/vanilla/deploy-vsphere-csi-validation-webhook.sh --help
now exits 0

bash manifests/vanilla/deploy-vsphere-csi-validation-webhook.sh --namespace vmware-system-csi
still follows the normal path, and in this env stops at kubectl not found

Special notes for your reviewer:

No matching issue found, so this is just a tiny paper-cut fix.

Release note:

NONE

[kubernetes-prow]

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[immanuwell]

@divyenpatel @xing-yang please take a look