DEV: Add last_message_id to channel and thread (#22488)

Initial migration and changes to models as well as
changing the following services to update last_message_id:

* Chat::MessageCreator
* Chat::RestoreMessage
* Chat::TrashMessage

The data migration will set the `last_message_id` for all existing
threads and channels in the database.

When we query the thread list as well as the channel,
we look at the last message ID for the following:

* Channel - Sorting DM channels, and channel metadata for the list of channels
* Thread - Last reply details for thread indicators and thread list

plugins/chat/spec/components/chat/message_creator_spec.rb

@@ -120,16 +120,14 @@ describe Chat::MessageCreator do
       }.to change { Chat::Message.count }.by(1)
     end
 
-    it "updates the channel’s last message date" do
-      previous_last_message_sent_at = public_chat_channel.last_message_sent_at
-
-      described_class.create(
-        chat_channel: public_chat_channel,
-        user: user1,
-        content: "this is a message",
-      )
-
-      expect(previous_last_message_sent_at).to be < public_chat_channel.reload.last_message_sent_at
+    it "updates the last_message for the channel" do
+      message =
+        described_class.create(
+          chat_channel: public_chat_channel,
+          user: user1,
+          content: "this is a message",
+        ).chat_message
+      expect(public_chat_channel.reload.last_message).to eq(message)
     end
 
     it "sets the last_editor_id to the user who created the message" do
@@ -640,6 +638,18 @@ describe Chat::MessageCreator do
         expect(message.in_reply_to.thread).to eq(message.thread)
         expect(message.thread.original_message).to eq(reply_message)
         expect(message.thread.original_message_user).to eq(reply_message.user)
+        expect(message.thread.last_message).to eq(message)
+      end
+
+      it "does not change the last_message of the channel for a thread reply" do
+        original_last_message = public_chat_channel.last_message
+        described_class.create(
+          chat_channel: public_chat_channel,
+          user: user1,
+          content: "this is a message",
+          in_reply_to_id: reply_message.id,
+        )
+        expect(public_chat_channel.reload.last_message).to eq(original_last_message)
       end
 
       it "creates a user thread membership" do
@@ -756,6 +766,7 @@ describe Chat::MessageCreator do
           }.not_to change { Chat::Thread.count }
 
           expect(message.reload.thread).to eq(existing_thread)
+          expect(existing_thread.reload.last_message).to eq(message)
         end
 
         it "creates a user thread membership if one does not exist" do