Fix data race in garbage collector on node.owners field

[grandeit]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Add ownersLock to protect concurrent access to node.owners between GraphBuilder.processGraphChanges() (writer) and GC worker goroutines reading in blockingDependents() and unblockOwnerReferences() methods.

Also fix concurrent reads in the HTTP debug handler (/graph endpoint) for owners, dependents, beingDeleted, deletingDependents, and virtual fields by using their respective thread-safe accessor methods.

Which issue(s) this PR is related to:

Fixes: #135621

Special notes for your reviewer:

The fix follows the existing locking pattern in the node struct (dependentsLock, beingDeletedLock, etc.). GraphBuilder's own reads of node.owners remain as direct field access since it is single-threaded and the only writer.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[k8s-ci-robot]

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[k8s-ci-robot]

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: grandeit
Once this PR has been reviewed and has the lgtm label, please assign jpbetz for approval. For more information see the Code Review Process.

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Needs approval from an approver in each of these files:

• pkg/controller/garbagecollector/OWNERS

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[grandeit]

/cc @jpbetz @liggitt