fix: suppress pointer-events on overlays slotted content (#12471) (CP: 25.2)

[vaadin-bot]

This PR cherry-picks changes from the original PR #12471 to branch 25.2.

---

Original PR description

Description

Fixes #12465

• Added suppressed - internal attribute on vaadin-overlay, intentionally undocumented for styling.
• Changed OverlayStackMixin to set the new attribute rather than modifying inline style directly
• Adapted CSS block with closing in animation styles to use suppressed, moved it to base styles
• Updated Lumo overlay.css mixin accordingly - some overlays like dialog are not using base styles
• Improved IT added in fix: always close select on outside click when in modeless dialog #8727

Type of change

• Bugfix

How to Test

Update dev/dialog.html - make both dialogs modeless, add snippet below to dialog2.renderer:

const select = document.createElement('vaadin-select');
select.label = 'Select';
select.innerHTML = `
  <vaadin-select-list-box slot="overlay">
    <vaadin-select-item value="1">Option 1</vaadin-select-item>
    <vaadin-select-item value="2">Option 2</vaadin-select-item>
    <vaadin-select-item value="3">Option 3</vaadin-select-item>
  </vaadin-select-list-box>
`;

Steps to verify the fix works:

1. Open parent dialog, then nested dialog, then its nested select
2. Click on parent dialog header title or the footer "Action" button
3. Before: parent dialog is focused. After: only select is closed

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud