feat: Expected Runtime Plugin for Soft Eviction via Requeue Action

[rich7420]

Description

Adds the Expected Runtime plugin: running jobs that exceed their configured expected runtime get nominated as requeue candidates. The plugin only does nomination; eviction is done by the Requeue action (elsewhere). Soft eviction: jobs become eligible when runtime ≥ expected, but are only evicted when a higher-priority workload needs the slot.

Why: Time-aware fairness (requeue only when there’s contention), opt-in via kai.scheduler/expected-runtime, cooldown via kai.scheduler/requeue-not-before to avoid thrashing.

What changed:

• Plugin expectedruntime: registers RequeueCandidateNominationFn, nominates jobs that pass checks (running, preemptible, valid expected-runtime, runtime ≥ expected, cooldown expired).
• Session API: RequeueCandidateNominationFn, AddRequeueCandidateNominationFn, CollectRequeueCandidates() (dedup by PodGroup UID).
• Annotations: kai.scheduler/expected-runtime, requeue-delay, requeue-not-before.
• Metrics: kai_requeue_nominations_total, kai_requeue_nomination_skipped_total (prefix from --metrics-namespace, default kai).
• Operator: expectedruntime in default plugin list; docs in docs/plugins/expectedruntime.md.

Uses existing LastStartTimestamp; MinRuntime stays in Requeue action filters.

Related Issues

Closes #904

Checklist

Note: Ensure your PR title follows the Conventional Commits format (e.g., feat(scheduler): add new feature)

• Self-reviewed
• Added/updated tests (if needed)
• Updated documentation (if needed)

Breaking Changes

Additional Notes

[rich7420]

cc @itsomri , @romanbaron