Move `ask_questions` tool implementation into VS Code core

[karthiknadig]

Per feedback in #291327, the ask_questions tool (question carousel) has tight coupling with UI and returns a value from ChatAgentResponseStream, which is contrary to the "stream" concept.

The tool should be implemented similar to the demo extension approach, with the implementation living in VS Code core rather than requiring the extension to await a response from the stream.

Related:

• Pressing Stop does not stop Agent session if using Ask tool #291295 (original stop button issue)
• fix: add cancellation token support to ChatAgentResponseStream #291327 (cancellation fix PR with this feedback)

cc @roblourens @digitarald

[roblourens]

Thanks, I think tools with tight coupling to the VS Code UI should live in VS Code. Similar to the todo tool, terminal tool, etc. Will make things much easier for copilot!