This PR tweaks the message role indicator in the assistant panel.

Previously we added a negative margin in #3925 to align the indicator
with the text, but the result of this was the left side of the button
getting clipped on hover.

To remedy this, we now use the filled button style so that the
background always shows and maintains the visual alignment.

### Before

<img width="451" alt="Screenshot 2024-01-08 at 10 50 21 AM"
src="https://github.com/zed-industries/zed/assets/1486634/dd3ae3dc-13c5-4d81-abd7-8023aca496c8">

### After

<img width="401" alt="Screenshot 2024-01-08 at 10 54 42 AM"
src="https://github.com/zed-industries/zed/assets/1486634/c089a879-1c88-4d94-9532-4e9433a35828">

We could also do what we did in Zed1, where the hover style doesn't have
any padding and is flush with the left side of the text, but I don't
think this looks as polished.

The "Assistant" role now also uses a separate color from "System".

Release Notes:

- Updated the style of the message role indicator in the assistant
panel.