Fix StatefulSet ordinal election leaving buckets unowned

[shashankxrm]

Changes

• 🐛 Reject StatefulSet ordinal leader election when the replica count does not match the configured bucket count, instead of starting with unowned buckets.
• 🧹 Treat STATEFUL_CONTROLLER_ORDINAL as the explicit selector for StatefulSet mode. Invalid explicit configuration now fails closed instead of falling back to lease election. Standard lease election is unchanged when the ordinal env is absent.

StatefulSet assignment remains 1:1 (ordinal N owns bucket N). Bucket names, DNS identities, and valid StatefulSet election behavior are unchanged.

/kind bug

Fixes #3384

Release Note

action required: StatefulSet ordinal leader election now requires STATEFUL_REPLICA_COUNT to equal the configured bucket count. Invalid explicit StatefulSet configuration fails closed instead of falling back to lease election.

Docs

[knative-prow]

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[knative-prow]

Welcome @shashankxrm! It looks like this is your first PR to knative/pkg 🎉

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