Stop breadcrumb overflow toggle from showing hidden panel header

What does this MR do and why?

Follow-up to !254742 (merged), addressing !254742 (comment 3941068440).

!254742 (merged) broadened the :has() rule on .panel-header-inner.without-breadcrumbs in app/assets/stylesheets/layouts/panel.scss from direct children (:has(> :is(.gl-badge, .gl-button))) to any descendant (:has(.gl-badge, .gl-button)). The breadcrumbs partial is always rendered inside the hidden header. While it measures, GlBreadcrumb renders its "…" overflow toggle, a .gl-button, and the broadened rule matches it. The header then shows, the breadcrumbs fit, the toggle goes away, and the header hides again. This loop happens whenever page_breadcrumbs_in_top_bar is enabled, which makes feature specs flaky (https://gitlab.com/gitlab-org/quality/test-failure-issues/-/work_items/44876).

This MR scopes the rule to:

• Direct children (> :is(.gl-badge, .gl-button)): keeps the previous behavior, for example the external authorization classification badge from layouts/nav/_classification_level_banner.html.haml.
• The actions container (.panel-header-inner-actions :is(.gl-badge, .gl-button)): keeps the nested match that PanelActionsPortal needs, which pages like the job page (!258490 (merged)) rely on when they don't set @force_show_panel_header.

Buttons inside the breadcrumbs no longer cause the header to show.

How to set up and validate locally

1. Enable the flag: Feature.enable(:page_breadcrumbs_in_top_bar).
2. Open a page that doesn't set @force_show_panel_header, for example a project's merge request list. The top edge of the panel no longer flickers, and the header stays hidden.
3. Open a merge request. The panel header actions still appear.
4. Optionally, with !258490 (merged) applied, open a job page. The portal actions still show the header.

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