Reconfigurator: Planner should check disks and datasets before decommissioning a sled

#7234 implements an internal validate_decommissionable() method on SledEditor that checks whether a sled the planner wants to decommission has any non-expunged zones. It should also check for non-expunged disks and datasets, but at the moment doing so causes a variety of test failure. We should add something like this:

    fn validate_decommisionable(&self) -> Result<(), SledEditError> {
        // TODO-john The disks and datasets checks below don't pass what the
        // planner does currently to decommission sleds: if a sled is expunged,
        // we'll omit its disks and datasets from the outgoing blueprint
        // entirely without setting them all to the `Expunged` disposition.
        // Fixing this will conflict with ongoing disk work, so for now these
        // checks are commented out.
        /*
        // Check that all disks are expunged...
        if let Some(disk) =
            self.disks(DiskFilter::All).find(|disk| match disk.disposition {
                BlueprintPhysicalDiskDisposition::InService => true,
                BlueprintPhysicalDiskDisposition::Expunged => false,
            })
        {
            return Err(SledEditError::NonDecommissionableDiskInService {
                disk_id: disk.id,
                zpool_id: disk.pool_id,
            });
        }

        // ... and all datasets are expunged ...
        if let Some(dataset) =
            self.datasets(BlueprintDatasetFilter::All).find(|dataset| {
                match dataset.disposition {
                    BlueprintDatasetDisposition::InService => true,
                    BlueprintDatasetDisposition::Expunged => false,
                }
            })
        {
            return Err(SledEditError::NonDecommissionableDatasetInService {
                dataset_id: dataset.id,
                kind: dataset.kind.clone(),
            });
        }
        */

        // ... rest of method to check zones ...
    }

Start with SledEditor::validate_decommissionable and the implementation from #7234, then inspect the planner's outgoing blueprint handling for expunged sleds. Restore the disk and dataset checks and run the affected planner tests; done means non-expunged disks and datasets prevent decommissioning without breaking existing sled decommissioning behavior.