Add a "Copy response" quick-action button to message actions

Other CLI/agentic tools (e.g. Codex/ChatGPT orchestrator UIs) show a dedicated "Copy response" button directly in the action bar under each assistant message, making it fast to copy the full response and hand it to an external orchestrator or another tool. It would help workflows where a user chains multiple agents/tools together (e.g. copying Claude Code's output and feeding it into ChatGPT as an orchestrator) if Claude Code's VS Code extension / CLI had a similarly prominent one-click "copy full response" action, distinct from copying individual code blocks.