FIX: My Thread's last reply excerpt on mobile (#27072)

... wasn't properly escaped so it would should html entities (like `'` instead of the apostrophe `'`).

I checked all the other places we show an excerpt and this was the only one that was missing the call to `htmlSafe` -> `replaceEmoji`.

Internal ref - t/128877

plugins/chat/spec/system/user_threads_spec.rb

@@ -245,7 +245,13 @@ RSpec.describe "User threads", type: :system do
 
   context "when in mobile", mobile: true do
     before do
-      chat_thread_chain_bootstrap(channel: channel_1, users: [current_user, Fabricate(:user)])
+      last_message =
+        chat_thread_chain_bootstrap(
+          channel: channel_1,
+          users: [current_user, Fabricate(:user)],
+        ).last_message
+
+      update_message!(last_message, text: "How's everyone doing?")
     end
 
     it "has the expected UI elements" do
@@ -258,6 +264,8 @@ RSpec.describe "User threads", type: :system do
       expect(user_threads_page).to have_css(".c-user-thread__excerpt")
       expect(user_threads_page).to have_css(".c-user-thread__excerpt-poster")
       expect(user_threads_page).to have_css(".c-user-thread .relative-date")
+
+      expect(user_threads_page.excerpt_text).to eq("How's everyone doing?")
     end
   end
 end