Issue #3585024: Warn when "Notification User Before Deletion" is disabled with "Users pre-notified only" condition active

Closes #3585024.

Summary

Adds a validation warning when a Purge Users policy combines the "Users pre-notified only" condition (with notification_required enabled) with a pre-deletion notification that is effectively disabled. In that configuration, no user can ever be purged because the condition requires a prior notification record.

Changes

• PurgeUsersPolicyEditForm::hasNotificationConflict() detects the conflict by looking up the purge_users:notification_required condition and evaluating send_email_user_before_notification (including the default cascade to the site-wide setting).
• When the conflict is detected on form load, the Notification User Before Deletion details section is expanded and an inline messages--warning box is shown.
• When the policy is saved with the conflict, a messenger()->addWarning() is also emitted so the admin sees the message after redirect.