`last_message_sent_at` has a `NOT_NULL` constraint in the DB so it should be safe to use for sorting.
This was causing two flakeys:
```
1) UserNotifications.chat_summary with public channel email subject with regular mentions includes both channel titles when there are exactly two with unread mentions
Failure/Error: example.run
expected: "[Discourse] New message in Random 62 and Test channel"
got: "[Discourse] New message in Test channel and Random 62"
(compared using ==)
# ./plugins/chat/spec/mailers/user_notifications_spec.rb:203:in `block (6 levels) in <main>'
# ./spec/rails_helper.rb:356:in `block (2 levels) in <top (required)>'
# ./vendor/bundle/ruby/3.1.0/gems/webmock-3.18.1/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'
2) UserNotifications.chat_summary with public channel email subject with regular mentions displays a count when there are more than two channels with unread mentions
Failure/Error: example.run
expected: "[Discourse] New message in Random 62 and 2 others"
got: "[Discourse] New message in Test channel 0 and 2 others"
(compared using ==)
# ./plugins/chat/spec/mailers/user_notifications_spec.rb:236:in `block (6 levels) in <main>'
# ./spec/rails_helper.rb:356:in `block (2 levels) in <top (required)>'
# ./vendor/bundle/ruby/3.1.0/gems/webmock-3.18.1/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'
```
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| .. | ||
| chat | ||
| discourse-details | ||
| discourse-local-dates | ||
| discourse-narrative-bot | ||
| discourse-presence | ||
| lazy-yt | ||
| poll | ||
| styleguide | ||