Caseflow: Stop Tasks from being `assign_to_type` `Vso`

[lomky]

Description

Tasks should have the assigned_to_type value of either User or Organization, however we discovered a handful assigned to Vso.

Background/context/resources

• Slack thread fixing the previously found ones

Technical notes

Acceptance Criteria

• [ ] Integrity check created enforcing only assign_to_types of User and Organization
  
  
  
  • [ ] If that is not possible, create an alert for any other value
  
  

[lowellrex]

We could alternatively put a constraint on the database to only those two values (enum(Organization, User)).

[yoomlam]

Example of need for #12773.

[hschallhorn]

Rather than a data integrity check, let's go this route

_What is this graph?_

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Seems fairly straightforward. Could be as simple as adding one line to the model and a test. Kat is clutching her ~pearls~ ones. Triple check that all tasks have type User or Org.