External Email Participants Not Receiving Comment Notifications on Public Projects (v18.0.2-ee)

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The /add_email feature for External Participants is partially broken. The initial "You have been added" email is sent and received successfully, but the external participant does not receive any notifications for subsequent public comments, even though the UI indicates they will.

Steps to reproduce:

• Create a new project and set its visibility to Public.
• Create a new issue.
• Have User A (an admin or the issue creator) post a comment with /add_email <external_address>.
• Observe: The external address receives the "You have been added" email.
• Have User A post a second, public comment.
• Expected Result: The external address should receive a notification about the new comment.
• Actual Result: No email is sent. There is no trace of the email address or a mailer job in either the sidekiq or production_json logs during this step.
• Have a different user (User B) post a public comment.
• Actual Result (from your test): No email is sent in this case either, confirming the issue is not related to the "self-notification" rule.