A failed agent format (e.g. 502 'device is mounted', ok:false, data:null) fell
through FormatDisk's trailing 'return out, nil', so the web layer reported a
zero-value FormatResult as ok:true — a failed DESTRUCTIVE format read as success.
postWithStatus now returns the full envelope; FormatDisk returns a non-nil error
on any non-2xx/ok:false that is not a recognized refusal (403/needs-confirmation).
Test TestFormat_MountedFailureSurfacesError (502 → non-nil err) fails on old code.
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