assistant2: Tweak the settings UI (#23845)

This PR does some somewhat light UI adjustment to the Assistant 2
settings view. The Prompt Library section should feature the default
prompts in the future, so that's why it's been separated that way.

<img width="800" alt="Screenshot 2025-01-29 at 2 59 59 PM"
src="https://github.com/user-attachments/assets/7b033bde-51ab-44d5-9e53-3f72b8ff5f51"
/>

Release Notes:

- N/A

crates/language_models/src/provider/google.rs

@@ -452,7 +452,7 @@ impl Render for ConfigurationView {
                 .child(h_flex().child(Label::new(INSTRUCTIONS[1])).child(
                     Button::new("google_console", GOOGLE_CONSOLE_URL)
                         .style(ButtonStyle::Subtle)
-                        .icon(IconName::ExternalLink)
+                        .icon(IconName::ArrowUpRight)
                         .icon_size(IconSize::XSmall)
                         .icon_color(Color::Muted)
                         .on_click(move |_, _, cx| cx.open_url(GOOGLE_CONSOLE_URL))
@@ -466,6 +466,8 @@ impl Render for ConfigurationView {
                         .px_2()
                         .py_1()
                         .bg(cx.theme().colors().editor_background)
+                        .border_1()
+                        .border_color(cx.theme().colors().border_variant)
                         .rounded_md()
                         .child(self.render_api_key_editor(cx)),
                 )