When evacuating or rebalancing EC shards, pickBestDiskOnNode now
filters disks by the target disk type. This ensures:
1. EC shards from SSD disks are moved to SSD disks on destination nodes
2. EC shards from HDD disks are moved to HDD disks on destination nodes
3. No cross-disk-type shard movement occurs
This maintains the storage tier isolation when moving EC shards
between nodes during evacuation or rebalancing operations.