fix(scheduler): correct disk-level storage double counting during replica scheduling

[davidcheng0922]

Which issue(s) this PR fixes:

longhorn/longhorn#12653

What this PR does / why we need it:

Fixes incorrect disk-level storage accounting during replica scheduling.

Previously, when scheduling multiple replicas on the same node but different disks, the scheduler incorrectly counted replica sizes across disks. This could lead to false "insufficient storage" errors even when the target disk had enough available space.

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Pull request overview

This PR fixes a critical bug in the replica scheduler where disk-level storage was being incorrectly double-counted during replica scheduling. When scheduling multiple replicas on the same node but different disks, the scheduler incorrectly counted replica sizes across all disks on a node instead of per-disk, potentially causing false "insufficient storage" errors even when the target disk had enough space.

Changes:

• Added disk-level filtering (r.Spec.DiskID == diskUUID) to storage accounting logic during replica scheduling
• Improved code comment to clarify the purpose of accounting for replicas already assigned during the current scheduling cycle

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