Reject duplicate init container names and report them under initContainers

[arpitjain099]

Fixes #

Proposed Changes

• validateInitContainers seeds allNames from the regular containers and never adds the init container names it walks past, so two init containers sharing a name are accepted. validateContainers has the else { allNames.Insert(...) } that makes the same check work for regular containers. Issue Container names are not validated for duplicates #12260 said in its first line that this affects init containers too; PR Fix container name collision #12374 fixed the init-vs-regular half and tested that half only.
• The errors from validateInitContainers use ViaFieldIndex("containers", i) and the one caller does not wrap in ViaField("initContainers"), so an init container fault comes back as containers[0].lifecycle or containers[0].securityContext, pointing the user at a different container than the one at fault.
• The existing "init-container name collision" case expected the wrong path, so it moves to initContainers[0] along with the fix, and there is a new case for two init containers with the same name.

The new case fails on main with want "duplicate container name \"same\": initContainers[1].name", got "". go test ./pkg/apis/serving/... passes with the change.

Release Note

Duplicate init container names are now rejected, and validation errors for init containers report the initContainers field path instead of containers.

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Welcome @arpitjain099! It looks like this is your first PR to knative/serving 🎉

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