fix(sidecar-controller): retry conflict when clearing content status

[TOGEP]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change

/kind bug

/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
This PR adds RetryOnConflict handling to the sidecar delete flow.

Specifically, it ensures that clearing VolumeSnapshotContent status
after CSI delete succeeds is retried on transient conflicts.

Changes include:

• clearVolumeContentStatus now uses retry.RetryOnConflict.
• Re-fetch the latest VolumeSnapshotContent on each retry before calling UpdateStatus.

Tests:

• Updated delete sync tests in pkg/sidecar-controller/snapshot_delete_test.go.
• Added case 1-3b to verify that a transient UpdateStatus conflict is retried and eventually succeeds.
• Conflict injection uses Kubernetes conflict errors (apierrors.NewConflict) to align with RetryOnConflict behavior.
• Added inline comment clarifying expected finalizer removal after status clear succeeds in the delete-policy flow.

Which issue(s) this PR fixes:

Related to #748

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

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