fix: handle PodGroup CRD tombstone deletion

[0YHR0]

What type of PR is this?

/kind bug

What this PR does / why we need it

The PodGroup CRD informer assumed every delete notification contained a direct CustomResourceDefinition object. Kubernetes informers can instead deliver cache.DeletedFinalStateUnknown tombstones when the object is no longer present in the local cache, causing the unchecked type assertion to panic.

This change moves delete handling into a dedicated helper that:

• handles direct CRD delete events;
• unwraps tombstone delete events before updating PodGroup support state;
• logs and ignores unexpected object types instead of panicking.

Regression tests cover direct deletes, tombstone deletes, unexpected objects, and tombstones containing unexpected objects.

Which issue(s) this PR fixes

None.

Special notes for your reviewer

The change only affects the PodGroup CRD informer delete path. Add and update behavior is unchanged.

How was this change tested?

go test ./pkg/model-serving-controller/podgroupmanager -run 'TestHandlePodGroupCRD(Delete|Change)$' -count=1
go test ./pkg/model-serving-controller/podgroupmanager -count=1

Does this PR introduce a user-facing change?

NONE

[gemini-code-assist]

Caution

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[volcano-sh-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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Details

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[0YHR0]

/retest

[volcano-sh-bot]

@0YHR0: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

Details

In response to this:

/retest

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[kube-gopher]

/lgtm