KEP-4438: align alpha KEP with the merged implementation

[matthyx]

PR-structure-exempt: targets kubernetes/enhancements, which uses the "KEP-: ..." title convention rather than conventional-commit

Summary

Aligns the KEP-4438 alpha document with the implementation merged in kubernetes/kubernetes#140133 — corrected alpha limitations, filled-in Design Details, updated test/monitoring references, and refreshed the Beta criteria.

Changes

• Alpha limitations corrected — restarted sidecars now receive an ordered SIGTERM (not SIGKILL at sandbox teardown); the restart is not observable via pod status, so a kubelet_sidecar_restarts_during_termination_total metric is exposed instead; preStop runs once at t=0 on the instance that was live at the start of termination; added the no-backoff and ~1s-polling limitations.
• Design Details filled in (was previously empty) with the implemented approach.
• Test Plan unit- and e2e-test references updated to the tests that were added.
• Monitoring documents the new metric.
• Beta graduation criteria and Implementation History updated.

The feature-gate name in the code was renamed to match this KEP (SidecarsRestartableDuringPodTermination).

Opening as a draft for author/reviewer review.

Drafted with the help of AI.

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[kubernetes-prow]

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• ✅ login: matthyx / name: Matthias Bertschy (6b1bdff)