Update pickBestDiskOnNode to accept a strictDiskType parameter:
- strictDiskType=true (balancing): Only use disks of matching type.
This maintains storage tier isolation during normal rebalancing.
- strictDiskType=false (evacuation): Prefer same disk type, but
fall back to other disk types if no matching disk is available.
This ensures evacuation can complete even when same-type capacity
is insufficient.
Priority order for evacuation:
1. Same disk type with lowest shard count (preferred)
2. Different disk type with lowest shard count (fallback)
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