chore(ui5-combobox): improve accessibility and interaction when items are loaded with delay

[nikoletavnv]

The change enhances screen reader announcement when a loading of items starts and ends. It also introduces a new event 'load-items' that users can use in order to set the ui5-combobox or ui5-multi-combobox component in a loading state.

JIRA: BGSOFUIRILA-4491

[ui5-webcomponents-bot]

🚀 Deployed on https://pr-13507--ui5-webcomponents-preview.netlify.app

[ivoplashkov]

• MCB fires load-items with the old value. In _inputLiveChange, fireOnInput() runs at the top, but this.value isn't updated from the input until later — so detail.value is one keystroke behind. ComboBox does it the other way (fires after updating value), so the two components disagree.
• Announced count doesn't match what's shown. MCB counts group headers as "results" (ComboBox correctly excludes them), and the count comes from visible items rather than the filtered list that was just set in onLoadingEnd — so we can announce "5 results" while the list shows 2.
• The announcement always says "Data loaded" even if the fetch threw. I see that this might be impossible to cover as a use case, but maybe we can discuss whether we should leave the "Data loaded" announcement to be on app side as well.