When dismissing an admin notice, the service would look up the
associated `ProblemCheckTracker` by identifier only. This could find a
tracker with a `NULL` target, which then fails validation on update
since target is now required (as of 8ca5fb706a).
The root issue is that `problem_check_trackers.target` was added with
`null: true` and while we later added a default value of `"__NULL__"`
and a Ruby validation, we never enforced `NOT NULL` at the database
level. This allowed records with `NULL` targets to persist or be
recreated through code paths that explicitly passed nil.
This commit:
- Updates the dismiss service to look up trackers by both identifier AND
target (extracted from the admin notice's details)
- Removes any remaining records with `NULL` targets
- Adds a `NOT NULL` constraint to prevent this from recurring
Ref - https://meta.discourse.org/t/392248
ae58a5a1be
