Add support for UI request to close itself

[fredericbarthelet]

There is window.openai.requestClose method available in widget for ChatGPT App.
On MCP App, there is modelcontextprotocol/ext-apps#203 and modelcontextprotocol/ext-apps#215

We could implement a unified API const { widgetId, terminate } = useWidget() ?

[harijoe]

Both referenced PR have been merged. Tagging this as good first issue.