AO3-5696 fix orphan account chapter subscription emails

[pmonfort]

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Issue

https://otwarchive.atlassian.net/browse/AO3-5696

Purpose

When orphan_account is a co-creator on a multi-chapter work and another co-creator posts a new chapter without listing orphan_account, users subscribed to orphan_account incorrectly receive subscription emails.

The existing check in Subscription#valid_notification_entry? only inspects the creation's (chapter's) pseuds, missing the case where orphan_account is a co-creator at the work level but not on the individual chapter.

Added a guard that skips notifications when the subscription is to orphan_account directly. Subscribers of other co-creators on the same work are not affected.

Testing Instructions

1. Subscribe to orphan_account
2. Create a work with orphan_account as co-creator
3. Post a new chapter without adding orphan_account as chapter co-creator
4. Run bundle exec rake notifications:deliver_subscriptions
5. Verify no subscription email is sent to the subscriber of orphan_account
6. Verify that subscribers of the other co-creator still receive emails

References

Related to AO3-3620 (Allow subscriptions to orphan_account to receive notifications). When AO3-3620 is fixed, the two
User.orphan_account guards in valid_notification_entry? should be removed.

Credit

Pablo Monfort (he/him)