fix(ui5-toolbar-button): ensure getDomRef() returns the actual DOM reference

[kgogov]

This commit fixes the issue where getDomRef() on ui5-toolbar-button was returning undefined by providing a custom implementation that correctly returns the DOM reference to the actual ui5-button element rendered within the ui5-toolbar's shadow DOM. This enhancement ensures that developers can reliably interact with the toolbar button's DOM element.

Notes:

• This fix assumes that ui5-toolbar-button is a direct child of ui5-toolbar.
• If the component hierarchy changes, additional checks may be necessary to ensure the parent toolbar is correctly identified.

Fixes: #9996