FIX: Chat summary email link in subfolder setups (#25188)

This regressed in 2791e75072329fdbf9ebd6849cea3c63664b123c. That commit
fixed subfolder URLs in general, but the `full_url` was adding the
subfolder prefix a second time, thus breaking this URL in emails.

plugins/chat/app/models/chat/message.rb

@@ -236,7 +236,7 @@ module Chat
     end
 
     def full_url
-      "#{Discourse.base_url}#{url}"
+      "#{Discourse.base_url_no_prefix}#{url}"
     end
 
     def url

plugins/chat/spec/mailers/user_notifications_spec.rb

@@ -637,6 +637,17 @@ describe UserNotifications do
             expect(user_avatar.attribute("alt").value).to eq(sender.username)
           end
 
+          context "with subfolder" do
+            before { set_subfolder "/community" }
+
+            it "includes correct view summary link in template" do
+              email = described_class.chat_summary(user, {})
+              expect(email.html_part.body.to_s).to include(
+                "<a class=\"more-messages-link\" href=\"#{Discourse.base_url}/chat",
+              )
+            end
+          end
+
           context "when there are more than two mentions" do
             it "includes a view more link " do
               2.times do