FEATURE: Reacting to MessageBus in chat thread panel (#21070)

This commit introduces a ChatChannelPaneSubscriptionsManager
and a ChatChannelThreadPaneSubscriptionsManager that inherits
from the first service that handle MessageBus subscriptions
for the main channel and the thread panel respectively.

This necessitated a change to Chat::Publisher to be able to
send MessageBus messages to multiple channels based on whether
a message was an OM for a thread, a thread reply, or a regular
channel message.

An initial change to update the thread indicator with new replies
has been done too, but that will be improved in future as we have
more data to update on the indicators.

Still remaining is to fully move over the handleSentMessage
functionality which includes scrolling and new message indicator
things.

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

spec/system/page_objects/cdp.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+module PageObjects
+  class CDP
+    include Capybara::DSL
+
+    def allow_clipboard
+      cdp_params = {
+        origin: page.server_url,
+        permission: {
+          name: "clipboard-read",
+        },
+        setting: "granted",
+      }
+      page.driver.browser.execute_cdp("Browser.setPermission", **cdp_params)
+
+      cdp_params = {
+        origin: page.server_url,
+        permission: {
+          name: "clipboard-write",
+        },
+        setting: "granted",
+      }
+      page.driver.browser.execute_cdp("Browser.setPermission", **cdp_params)
+    end
+
+    def read_clipboard
+      page.evaluate_async_script("navigator.clipboard.readText().then(arguments[0])")
+    end
+  end
+end