fix(ui5-toolbar): prevent flash of AlwaysOverflow items on first render

[kineticjs]

Problem:
Items with overflow-priority="AlwaysOverflow" would briefly appear in the
main toolbar before moving to the overflow popover. This occurred because
overflow distribution happened after rendering:

1. First render placed all items in standardItems (itemsToOverflow was empty)
2. After render, distributeItems() populated itemsToOverflow with AlwaysOverflow items
3. This state change triggered re-render, causing visible flash as items moved

This affected both initial render and dynamically added AlwaysOverflow items.

Solution:
Added prePopulateAlwaysOverflowItems() method called in onBeforeRendering()
that adds any AlwaysOverflow items not already in itemsToOverflow. This ensures:

• Initial render already places AlwaysOverflow items in overflow
• Dynamically added AlwaysOverflow items are handled before their first render
• Existing overflow calculation logic remains unchanged (items aren't duplicated)

Fixes: #12067