Fixes an issue introduced by #29959 which caused the message editor to
overflow from the agent panel bounds, making the bottom buttons
invisible when the editor was expanded (cmd+esc).

Fixing this required changing the base structure of the agent panel, but
things seem to work as expected:


https://github.com/user-attachments/assets/fc4c97fb-f7cb-4f54-a268-c30fbcb1649f


Release Notes:

- agent: Fix message editor's button positions when expanded