Add telemetry for chat todo list widget interactions

[bhavyaus]

Adds a chatTodoListWidget telemetry event to track user interactions with the todo list widget:

• expand — user expands the todo list
• collapse — user collapses the todo list
• clear — user clicks the clear button (X)

Each event includes the todoCount at the time of the action.

Note: the clear event only fires on explicit user action (button click), not when todos are programmatically cleared after completion.

[Copilot]

Pull request overview

Adds interaction telemetry for the chat todo list widget so product insights can measure how users engage with the widget (expand/collapse/clear) along with the current todo count.

Changes:

• Injects ITelemetryService into ChatTodoListWidget.
• Logs a new chatTodoListWidget telemetry event for expand, collapse, and clear actions with todoCount.
• Introduces event + GDPR classification types for the new telemetry event.

Comments suppressed due to low confidence (2)

src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatTodoListWidget.ts:375

• toggleExpanded() calls getTodos(this._currentSessionResource) twice (once to compute todoCount and again to update the title). Consider retrieving the todo list once and reusing it for both telemetry and title update to avoid duplicated work and keep the logic in sync.

This issue also appears on line 363 of the same file.

		const todoCount = this._currentSessionResource ? this.chatTodoListService.getTodos(this._currentSessionResource).length : 0;
		this.telemetryService.publicLog2<ChatTodoListWidgetEvent, ChatTodoListWidgetClassification>(
			'chatTodoListWidget',
			{
				action: this._isExpanded ? 'expand' : 'collapse',
				todoCount
			}
		);

		if (this._currentSessionResource) {
			const todoList = this.chatTodoListService.getTodos(this._currentSessionResource);
			this.updateTitleElement(this.titleElement, todoList);
		}

src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatTodoListWidget.ts:370

• Telemetry was added for expand/collapse/clear, but the existing ChatTodoListWidget browser tests don't cover these new events. Adding assertions (with a stubbed/test ITelemetryService) that the correct event name + payload are logged on (1) expando click, (2) clear click, and (3) focus-triggered expansion would prevent regressions and validate the "explicit user action only" requirement for clear.

		const todoCount = this._currentSessionResource ? this.chatTodoListService.getTodos(this._currentSessionResource).length : 0;
		this.telemetryService.publicLog2<ChatTodoListWidgetEvent, ChatTodoListWidgetClassification>(
			'chatTodoListWidget',
			{
				action: this._isExpanded ? 'expand' : 'collapse',
				todoCount
			}
		);