Chat Window UX Improvements

Hi there! 👋

I've been using the Copilot Chat window in Xcode and noticed a few things that could make the experience even better:

- When attaching files, I need to manually click into the search field to start typing. It would feel more natural if the search field automatically got focus when I click the attachment icon.
- I often find myself wanting to use the keyboard to select files from the search results, but currently I have to use the mouse. Adding keyboard navigation would make this much smoother!
- A small but annoying thing: when I copy code using the button at the bottom, it adds markdown code block formatting (```) to the copied text. It would be great if it just copied the clean code. I know that there is a copy button inside the code block, but in longer conversations, I often have to scroll up to reach it.
- The chat window keeps jumping to the bottom whenever it loses focus, which makes it tricky to read through longer conversations. It would be helpful if it maintained its scroll position.

These are all minor issues, but fixing them would make the chat experience feel much more polished and efficient.

Let me know if you need any additional information or would like me to clarify anything!

Environment:

Xcode Version: 16.2
GitHub Copilot Extension Version: 0.31.0
macOS Version: 15.3.1