assistant2: Add adjustments to OS notification (#27615)

This PR hardcodes the font size for the OS notification and adjusts the
copywriting on the `DoneStreaming` scenario.

1. Reason for the former change is because notifications always have a
fixed width and height, so any responsive design strategy here wouldn't
fully work.
2. Reason for the latter is because when the assistant response is done
streaming, that _can_ mean "changes have been applied" (previous label)
but it can also not mean that. So, I'm making it more generic now.

Release Notes:

- N/A

crates/assistant2/src/active_thread.rs

@@ -381,7 +381,7 @@ impl ActiveThread {
             ThreadEvent::DoneStreaming => {
                 if !self.thread().read(cx).is_generating() {
                     self.show_notification(
-                        "Your changes have been applied.",
+                        "The assistant response has concluded.",
                         IconName::Check,
                         Color::Success,
                         window,