Completed Scheduled Activity Snapshot Does Not Match Previous Snapshot

[jayfo]

Describe the Bug
Observed in v0.7.0, scheduled activities are being marked as completed with a snapshot that is not identical to their previous snapshot.

Expected Behavior
This should always be the case. Either:

• Something in the snapshot was modified before it was marked as completed, which should have generated a new snapshot.
• Or this was marking the scheduled activity completed, there's no reason for the snapshot to change.

Additional Context
Adding a warning in data validation to monitor this until it is fixed. See #427.

1. This is likely because the completed scheduled activity is generating an entirely new snapshot. Any scheduled activities that are overdue will not have been maintained and will still have their old snapshot, which therefore won't match the new snapshot.

   If that's what's happening, the expected fix is to simplify the "completed" path to use the previous snapshot.

2. I have now also seen this warning fired in validation of migrations from v0.6.0. So it is possible the current code is fine but these were introduced in migration.