PERF: Reduce overhead from chat message excerpt (#26712)

This change moves the chat message excerpt into a new database column (string) on the chat_messages table.

As part of this change, we will now set the excerpt within the `Chat::CreateMessage` service, and update it within the `Chat::UpdateMessage` service.

plugins/chat/spec/integration/outgoing_web_hooks_spec.rb

@@ -102,20 +102,12 @@ RSpec.describe "Outgoing chat webhooks" do
     context "for a category channel" do
       fab!(:category)
       fab!(:chat_channel) { Fabricate(:category_channel, chatable: category) }
-      fab!(:chat_message) { Fabricate(:chat_message, chat_channel: chat_channel, user: user1) }
-
-      before do
-        [user1, user2].each do |user|
-          Chat::UserChatChannelMembership.create(
-            user: user,
-            chat_channel: chat_channel,
-            following: true,
-          )
-        end
-
-        sign_in(user1)
+      fab!(:chat_message) do
+        Fabricate(:chat_message, use_service: true, chat_channel: chat_channel, user: user1)
       end
 
+      before { sign_in(user1) }
+
       it "triggers a webhook when a chat message is created" do
         post "/chat/#{chat_channel.id}.json", params: { message: message_content }
 
@@ -175,7 +167,12 @@ RSpec.describe "Outgoing chat webhooks" do
       fab!(:direct_message) { Fabricate(:direct_message, users: [user1, user2]) }
       fab!(:direct_message_channel) { Fabricate(:direct_message_channel, chatable: direct_message) }
       fab!(:chat_message) do
-        Fabricate(:chat_message, chat_channel: direct_message_channel, user: user1)
+        Fabricate(
+          :chat_message,
+          use_service: true,
+          chat_channel: direct_message_channel,
+          user: user1,
+        )
       end
 
       before { sign_in(user1) }