Adds a space after user mention to close out the auto-complete dialog

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/321855

ee/spec/features/epics/update_epic_spec.rb

@@ -84,7 +84,8 @@ RSpec.describe 'Update Epic', :js do
       it 'creates a todo only for mentioned users' do
         mentioned = create(:user)
 
-        fill_in 'issue-description', with: "FYI #{mentioned.to_reference}"
+        # Add a trailing space to close mention auto-complete dialog, which might block the save button
+        fill_in 'issue-description', with: "FYI #{mentioned.to_reference} "
 
         click_button 'Save changes'