assistant2: Sketch in context picker (#21560)

This PR sketches in a context picker into the message editor in
Assistant 2. Not functional yet.

<img width="1138" alt="Screenshot 2024-12-04 at 5 45 19 PM"
src="https://github.com/user-attachments/assets/053d6224-de76-4fde-914b-41fe835761eb">

Release Notes:

- N/A