Function validate_maintenance_admission_extras 

pub fn validate_maintenance_admission_extras(
    spec: &MaintenanceSpec,
) -> Vec<ValidationError>

Admission-only extras for a Maintenance: parse AND bound both jitter windows.

Unlike the other kinds, validate_maintenance never validated these fields at all — it covers the crons and the timezone only — so a garbage window has always been accepted and silently degraded to no jitter at reconcile. That means stored objects carrying garbage exist, and the parse half is a tightening too: both halves are admission-only, so those objects keep maintaining themselves (unspread) until someone edits them.