Respect DesiredScale in checkAndUpdateDeployment for scaled-to-zero services

[ayushh0311]

Fix scaled-to-zero services restarting on node preemption (re: #12538)

Modified checkAndUpdateDeployment to respect the PodAutoscaler's DesiredScale when available.

Fixes

#12538

Proposed Changes

• Modified checkAndUpdateDeployment in pkg/reconciler/revision/cruds.go to query the PodAutoscaler's DesiredScale status instead of blindly preserving the Deployment's current replica count
• During node disruptions, the revision reconciler now respects the autoscaler's scaling decisions, preventing scaled-to-zero services from restarting
• Added fallback to preserve deployment replicas when PodAutoscaler DesiredScale is unavailable (maintains backward compatibility)
• Added unit tests in pkg/reconciler/revision/cruds_test.go covering scale-to-zero preservation, scale-up scenarios, and backward compatibility

Release Note

Fixed an issue where scaled-to-zero Knative services would restart when spot/preemptible nodes were preempted and replaced. The revision reconciler now respects the PodAutoscaler's desired scale decision when updating deployments, preventing unintended pod recreation during node disruptions.

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[knative-prow]

Welcome @ayushh0311! It looks like this is your first PR to knative/serving 🎉

[knative-prow]

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[dprotaso]

Modified checkAndUpdateDeployment in pkg/reconciler/revision/cruds.go to query the PodAutoscaler's DesiredScale status instead of blindly preserving the Deployment's current replica count

The deployment's replica count is set by the PodAutoscaler's reconciler - so in my opinion the changes in this PR doesn't have any affect.

[dprotaso]

/hold

[dprotaso]

@ayushh0311 what observations do you have regarding these changes?