Add ChatResponseClearWidgetPart to enable programmatic widget clearing

[Copilot]

Extensions can now push a hidden ChatResponseClearWidgetPart to trigger chat widget clearing, useful for showing clean final results after intermediate progress messages.

Changes

Core Types

• chatService.ts: Added IChatClearWidget interface (kind: 'clearWidget')
• chatModel.ts: Integrated into IChatProgressResponseContent, excluded from history via nonHistoryKinds, ignored in text representation

Extension API

• vscode.d.ts: Exposed ChatResponseClearWidgetPart class
• extHostTypes.ts: Implemented API class
• extHost.protocol.ts: Added IChatClearWidgetDto for RPC
• extHostTypeConverters.ts: Added bidirectional conversion logic

Rendering

• chatListRenderer.ts: Added renderClearWidget() that retrieves widget via chatWidgetService.getWidgetBySessionResource() and calls widget.clear() asynchronously

Testing

• chatModel.test.ts: Verified clearWidget is ignored in response representation

Usage

const handler: vscode.ChatRequestHandler = async (request, context, stream, token) => {
    stream.markdown('Processing...');
    await performWork();
    
    // Clear intermediate messages
    stream.push(new vscode.ChatResponseClearWidgetPart());
    
    stream.markdown('# Final Results');
    return { metadata: {} };
};

The part renders nothing visible but triggers widget clearing when processed by the renderer.

Original prompt

Introduce a new 'hidden' chat part that extensions can push which tells the renderer (vscode) to clear the chat widget

Created from VS Code.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.