Behaviors are not attached on htmx swaps outside the builder root (side panels)

Problem/Motivation

When htmx swaps content, triggerDrupalBehaviorsFromHtmxEvent() looks for a
.display-builder ancestor and gives up when there is none. The side panels
live outside .display-builder in the DOM, so their content never gets
Drupal.attachBehaviors(). Any JavaScript behavior in a panel may never run:
an ajax button falls back to a native submit and the browser leaves the page
to /api/display-builder/..., dialogs never open. It looks intermittent
because another attach pass can still bind core ajax by element id.

This hits the new source selector from the ui_patterns issue #3528205: Performance: On-demand lazy-loading source selector: each prop has a button that opens a
dialog, both driven by behaviors. In the panel, the button triggers a navigation to the url instead of opening the dialog. But the bug is not specific to it: any behavior-based JavaScript in the panels seem affected.

Steps to reproduce

use code from ui patterns issue #3528205: Performance: On-demand lazy-loading source selector,
- Clear caches, open the display builder, select a component.
- In the right panel, click the button of a prop.
- The browser goes to /api/display-builder/... instead of staying on the page.

Proposed resolution

When the swap target has no .display-builder ancestor, still attach
behaviors, on the swapped element only:

    if (!root) {
      if (htmxLoadEvent.detail.elt) {
        Drupal.attachBehaviors(htmxLoadEvent.detail.elt, drupalSettings);
      }
      return;
    }