UX: Send notification of type `replied` to topic author if they're watching the topic (#18684)

Related to aeee7ed.

Before the change in aeee7ed, notifications for direct replies to your posts and notifications for replies in watched topics looked the same in the notifications menu -- they both used the arrow icon.

We decided in aeee7ed to distinguish them by changing "watched topics" notifications to use the bell icon because it was confusing for users who watch topics to see the same icon for direct replies and "watched topics". However, that change also means that non-power/new users who receive replies to topics _they create_ will get notifications with the bell icon because technically they're watching the topic, but the arrow icon is more appropriate for this case because we use it throughout the app to indicate "replies".

This commit adds a special-case so that if a user is watching a topic AND the topic is created by them, they receive notifications with the arrow icon (type `replied`) instead of the bell icon (type `posted`) for new posts in the topic.

Internal topic: t/79051.

app/services/post_alerter.rb

@@ -146,10 +146,17 @@ class PostAlerter
     # replies
     reply_to_user = post.reply_notification_target
 
-    if new_record && reply_to_user && !notified.include?(reply_to_user) && notify_about_reply?(post)
+    if new_record
+      if reply_to_user && !notified.include?(reply_to_user) && notify_about_reply?(post)
         notified += notify_non_pm_users(reply_to_user, :replied, post)
       end
 
+      topic_author = post.topic.user
+      if topic_author && !notified.include?(topic_author) && user_watching_topic?(topic_author, post.topic)
+        notified += notify_non_pm_users(topic_author, :replied, post)
+      end
+    end
+
     DiscourseEvent.trigger(:post_alerter_before_quotes, post, new_record, notified)
 
     # quotes
@@ -885,4 +892,12 @@ class PostAlerter
   def is_replying?(user, reply_to_user, quoted_users)
     reply_to_user == user || quoted_users.include?(user)
   end
+
+  def user_watching_topic?(user, topic)
+    TopicUser.exists?(
+      user_id: user.id,
+      topic_id: topic.id,
+      notification_level: TopicUser.notification_levels[:watching]
+    )
+  end
 end

spec/services/post_alerter_spec.rb

@@ -1322,6 +1322,27 @@ RSpec.describe PostAlerter do
       NotificationEmailer.expects(:process_notification).once
       expect { PostAlerter.post_created(reply) }.to change(user.notifications, :count).by(1)
     end
+
+    it "creates a notification of type `replied` instead of `posted` for the topic author if they're watching the topic" do
+      Jobs.run_immediately!
+
+      u1 = Fabricate(:admin)
+      u2 = Fabricate(:admin)
+
+      topic = create_topic(user: u1)
+
+      u1.notifications.destroy_all
+
+      expect do
+        create_post(topic: topic, user: u2)
+      end.to change { u1.reload.notifications.count }.by(1)
+      expect(u1.notifications.exists?(
+        topic_id: topic.id,
+        notification_type: Notification.types[:replied],
+        post_number: 1,
+        read: false
+      )).to eq(true)
+    end
   end
 
   context "with category" do