Adds diskhealthjson with its migration in the same change, so the model never references a column the schema has not applied.
Kept separate from disksjson rather than merged into it. That column is per volume and answers how full a drive is; this one is per physical device and answers whether the hardware is dying. They come from different WMI sources with different failure modes, so merging them would make a capacity collection failure indistinguishable from a health collection failure.
Nullable with no backfill: a drive whose health has never been reported is not a healthy drive, and defaulting to an empty list would read as no disks have problems on every endpoint predating the column.
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