DEV: Fix and improve chat system test for editing name and slug (#21810)

This commit introduces a couple of changes:

1. When editing a chat channel's slug, we were using `this.model.set("title", title)` when the `set`
   function does not exist. This was actually throwing the error in the
   "can edit slug" system test where the modal was not closed after
   saving and was flashing an error.

2. Introduce `PageObjects::Pages::ChatChannelAbout` and
   `PageObjects::Modals::ChatChannelEdit` page object to encapsulate
   logic better.

plugins/chat/spec/system/page_objects/modals/chat_edit_modal.rb

@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Modals
+    class ChatChannelEdit < PageObjects::Modals::Base
+      include SystemHelpers
+
+      EDIT_MODAL_SELECTOR = PageObjects::Pages::ChatChannelAbout::EDIT_MODAL_SELECTOR
+      SLUG_INPUT_SELECTOR = ".chat-channel-edit-name-slug-modal__slug-input"
+      NAME_INPUT_SELECTOR = ".chat-channel-edit-name-slug-modal__name-input"
+
+      def fill_in_slug(slug)
+        within(EDIT_MODAL_SELECTOR) { find(SLUG_INPUT_SELECTOR).fill_in(with: slug) }
+
+        self
+      end
+
+      def wait_for_auto_generated_slug
+        wait_for_attribute(page.find(SLUG_INPUT_SELECTOR), :placeholder, "test-channel")
+      end
+
+      def fill_in_slug_input(slug)
+        within(EDIT_MODAL_SELECTOR) { find(SLUG_INPUT_SELECTOR).fill_in(with: slug) }
+      end
+
+      def save_changes(successful: true)
+        within(EDIT_MODAL_SELECTOR) { click_button(I18n.t("js.save")) }
+
+        # ensures modal is closed after successfully saving
+        page.has_no_css?(EDIT_MODAL_SELECTOR) if successful
+      end
+
+      def fill_and_save_slug(slug)
+        fill_in_slug_input(slug)
+        save_changes
+        self
+      end
+
+      def fill_in_name_input(name)
+        within(EDIT_MODAL_SELECTOR) { find(NAME_INPUT_SELECTOR).fill_in(with: name) }
+      end
+
+      def fill_and_save_name(name)
+        fill_in_name_input(name)
+        save_changes
+        self
+      end
+
+      def has_slug_input?(value)
+        within(EDIT_MODAL_SELECTOR) { find(SLUG_INPUT_SELECTOR).value == value }
+      end
+
+      def has_name_input?(value)
+        within(EDIT_MODAL_SELECTOR) { find(NAME_INPUT_SELECTOR).value == value }
+      end
+    end
+  end
+end