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/deepseek.rs

@@ -506,7 +506,7 @@ impl Render for ConfigurationView {
                     h_flex().child(Label::new(INSTRUCTIONS[1])).child(
                         Button::new("deepseek_console", DEEPSEEK_CONSOLE_URL)
                             .style(ButtonStyle::Subtle)
-                            .icon(IconName::ExternalLink)
+                            .icon(IconName::ArrowUpRight)
                             .icon_size(IconSize::XSmall)
                             .icon_color(Color::Muted)
                             .on_click(move |_, _window, cx| cx.open_url(DEEPSEEK_CONSOLE_URL)),
@@ -520,12 +520,14 @@ 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)),
                 )
                 .child(
                     Label::new(format!(
-                        "Or set {} environment variable",
+                        "Or set the {} environment variable.",
                         DEEPSEEK_API_KEY_VAR
                     ))
                     .size(LabelSize::Small),