DEV: Add topic_tracking_state_publish_unread_scope modifier (#37236)

This modifier allows plugins to customize the TopicUser scope used when
publishing unread notifications via MessageBus.

Plugins can use this to filter which users receive unread notifications,
for example to exclude users who haven't visited the site in a certain
time period.

```
# in plugin.rb
DiscoursePluginRegistry.register_modifier(self, :topic_tracking_state_publish_unread_scope) do |scope, post|
  scope.joins(:user).where.not(user: { last_seen_at: nil })
end
```

app/models/topic_tracking_state.rb

@@ -149,6 +149,12 @@ class TopicTrackingState
     # this is why they are excluded from the initial scope.
     scope =
       TopicUser.tracking(post.topic_id).includes(user: :user_stat).where.not(user_id: post.user_id)
+    scope =
+      DiscoursePluginRegistry.apply_modifier(
+        :topic_tracking_state_publish_unread_scope,
+        scope,
+        post,
+      )
 
     group_ids =
       if post.post_type == Post.types[:whisper]

spec/models/topic_tracking_state_spec.rb

@@ -272,6 +272,31 @@ RSpec.describe TopicTrackingState do
         expect(messages).to eq([])
       end
     end
+
+    it "allows plugins to modify the scope via topic_tracking_state_publish_unread_scope modifier" do
+      user_to_exclude = Fabricate(:user)
+      Fabricate(:topic_user_watching, topic: topic, user: user_to_exclude)
+
+      messages = MessageBus.track_publish("/unread") { TopicTrackingState.publish_unread(post) }
+      expect(messages.first.user_ids).to include(user_to_exclude.id)
+
+      plugin = Plugin::Instance.new
+      modifier_block = Proc.new { |scope, _post| scope.where.not(user_id: user_to_exclude.id) }
+      DiscoursePluginRegistry.register_modifier(
+        plugin,
+        :topic_tracking_state_publish_unread_scope,
+        &modifier_block
+      )
+
+      messages = MessageBus.track_publish("/unread") { TopicTrackingState.publish_unread(post) }
+      expect(messages.first.user_ids).not_to include(user_to_exclude.id)
+    ensure
+      DiscoursePluginRegistry.unregister_modifier(
+        plugin,
+        :topic_tracking_state_publish_unread_scope,
+        &modifier_block
+      )
+    end
   end
 
   describe "#publish_muted" do