Ajax errors are hidden in expanded mode

Problem/Motivation

When a #ajax request fails, core's Drupal.AjaxError writes "Oops, something went wrong. Check your browser's developer console for more details." into the page's [data-drupal-messages] region. In expanded mode the builder covers the whole viewport, so the message is never seen and the action just seems to do nothing.

Steps to reproduce

• Open a builder and switch to expanded mode.
• Trigger a failing #ajax request from the Config panel, for example the one from #3623496: Config form: a rebuild carrying only its button replaces the source settings.
• No message appears; it is in the page's messages region, behind the builder.

Proposed resolution

Show Ajax errors in the builder's own toast stack, like server errors from RenderableBuilderTrait::buildError().

Core only creates Drupal.AjaxError.messages when none is set, so the builder sets its own, rendering the same alert into its stack, where its live region announces it.

A Playwright test expands the builder, raises the error and checks the toast is visible and not covered.