Fix EnsureWorkloadSlices to finish old slice when new is admitted as replacement

[sohankunkerkar]

When the scheduler admits a new workload slice but fails to finish the old slice, the job controller should clean it up. This fixes a race condition where old workload slices weren't being cleaned up during scale-up events,
causing the RayJob InTreeAutoscaling e2e test to see 4 workloads instead of 3.

What type of PR is this?

/kind bug
/kind flake

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #8447

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fix EnsureWorkloadSlices to finish old slice when new is admitted as replacement

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: sohankunkerkar
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[Copilot]

Pull request overview

This PR fixes a bug in the workload slice cleanup logic during elastic job scale-up operations. When the scheduler admits a new workload slice and sets the WorkloadSliceReplacementFor annotation, the old slice should be finished. Previously, the code only finished the old slice if it lost quota reservation or was evicted, missing the replacement annotation scenario.

Key Changes

• Added a third condition to finish old workload slices when the new slice has the replacement annotation pointing to it
• Added comprehensive inline documentation explaining the three conditions for finishing old slices
• Added test coverage for the replacement annotation scenario

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkg/workloadslicing/workloadslicing.go	Enhanced EnsureWorkloadSlices case 2 logic to check for replacement annotation and finish old slices accordingly
pkg/workloadslicing/workloadslicing_test.go	Added new test case "TwoWorkloads_OldWithReservedQuota_NewWithReplacementAnnotation" to verify the fix

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