When title or header elements received focus, they would trigger unwanted
scrolling because they are part of the scroll container but have static
scroll-padding. This caused scroll jumps when navigating between
content and header areas which lead to unsnap.

Step to reproduce:

1. Open https://ui5.github.io/webcomponents/nightly/packages/fiori/test/pages/DynamicPage.html
2. Scroll until title snaps
3. Place focus in content (e.g on a list item)
4. Press SHIFT+TAB multiple times to go through the buttons in the title

Result:
Page scroll up => title unsnaps

Demo solution on
https://pr-12588--ui5-webcomponents-preview.netlify.app/packages/fiori/test/pages/dynamicpage

Fix by implementing dynamic scroll padding that:

• Sets scroll-padding-top/bottom when focus enters content area
• Resets scroll-padding when focus leaves content area
• Prevents title/header focus from triggering scroll behavior

This ensures smooth focus transitions without unexpected scroll jumps
while maintaining proper scrollIntoView behavior for content elements.

Changes:

• Replace static scroll-padding with dynamic onContentFocusIn/onContentFocusOut handlers
• Remove lifecycle-based focus management (_focusInHandler, onAfterRendering/onExitDOM)
• Add topAreaHeight/endAreaHeight getters for dynamic padding calculation
• Add test coverage for focus handling and scroll padding behavior

Closes: #11689