FEATURE: Show unread in sidebar for unread channel threads (#22342)

This commit makes it so that when the user has unread threads
for a channel we show a blue dot in the sidebar (or channel index
for mobile/drawer).

This blue dot is slightly different from the channel unread messages:

1. It will only show if the new thread messages were created since
   the user last viewed the channel
2. It will be cleared when the user views the channel, but the threads
   are still considered unread because we want the user to click into
   the thread list to view them

This necessitates a change to the current user serializer to also
include the unread thread overview, which is all unread threads
across all channels and their last reply date + time.

plugins/chat/spec/queries/chat/tracking_state_report_query_spec.rb

@@ -9,6 +9,7 @@ RSpec.describe Chat::TrackingStateReportQuery do
       include_missing_memberships: include_missing_memberships,
       include_threads: include_threads,
       include_read: include_read,
+      include_last_reply_details: include_last_reply_details,
     )
   end
 
@@ -20,6 +21,7 @@ RSpec.describe Chat::TrackingStateReportQuery do
   let(:include_missing_memberships) { false }
   let(:include_threads) { false }
   let(:include_read) { true }
+  let(:include_last_reply_details) { false }
   context "when channel_ids empty" do
     it "returns empty object for channel_tracking" do
       expect(query.channel_tracking).to eq({})
@@ -129,6 +131,56 @@ RSpec.describe Chat::TrackingStateReportQuery do
         )
       end
 
+      context "when include_last_reply_details is true" do
+        let(:include_last_reply_details) { true }
+
+        before do
+          thread_1.add(current_user)
+          thread_2.add(current_user)
+          Fabricate(:chat_message, chat_channel: channel_1, thread: thread_1)
+          Fabricate(:chat_message, chat_channel: channel_2, thread: thread_2)
+        end
+
+        it "gets the last_reply_created_at for each thread based on the last_message" do
+          expect(query.thread_tracking).to eq(
+            {
+              thread_1.id => {
+                unread_count: 1,
+                mention_count: 0,
+                channel_id: channel_1.id,
+                last_reply_created_at: thread_1.reload.last_message.created_at,
+              },
+              thread_2.id => {
+                unread_count: 1,
+                mention_count: 0,
+                channel_id: channel_2.id,
+                last_reply_created_at: thread_2.reload.last_message.created_at,
+              },
+            },
+          )
+        end
+
+        it "does not get the last_reply_created_at for threads where the last_message is deleted" do
+          thread_1.reload.last_message.trash!
+          expect(query.thread_tracking).to eq(
+            {
+              thread_1.id => {
+                unread_count: 0,
+                mention_count: 0,
+                channel_id: channel_1.id,
+                last_reply_created_at: nil,
+              },
+              thread_2.id => {
+                unread_count: 1,
+                mention_count: 0,
+                channel_id: channel_2.id,
+                last_reply_created_at: thread_2.reload.last_message.created_at,
+              },
+            },
+          )
+        end
+      end
+
       context "when thread_ids and channel_ids is empty" do
         let(:thread_ids) { [] }
         let(:channel_ids) { [] }