assistant panel: Animate assistant label if message is pending (#16152)

This adds a pulsating effect to the assistant header in case the message
is pending.

The pulsating effect is capped between 0.2 and 1.0 and I tried (with the
help of Claude) to give it a "breathing" effect, since I found the
normal bounce a bit too much.

Also opted for setting the `alpha` on the `LabelLike` things, vs.
overwriting the color, since I think that's cleaner instead of exposing
the color and mutating that.


https://github.com/user-attachments/assets/4a94a1c5-8dc7-4c40-b30f-d92d112db7b5


Release Notes:

- N/A

crates/gpui/src/elements/animation.rs

@@ -170,6 +170,8 @@ impl<E: IntoElement + 'static> Element for AnimationElement<E> {
 }
 
 mod easing {
+    use std::f32::consts::PI;
+
     /// The linear easing function, or delta itself
     pub fn linear(delta: f32) -> f32 {
         delta
@@ -200,4 +202,20 @@ mod easing {
             }
         }
     }
+
+    /// A custom easing function for pulsating alpha that slows down as it approaches 0.1
+    pub fn pulsating_between(min: f32, max: f32) -> impl Fn(f32) -> f32 {
+        let range = max - min;
+
+        move |delta| {
+            // Use a combination of sine and cubic functions for a more natural breathing rhythm
+            let t = (delta * 2.0 * PI).sin();
+            let breath = (t * t * t + t) / 2.0;
+
+            // Map the breath to our desired alpha range
+            let normalized_alpha = (breath + 1.0) / 2.0;
+
+            min + (normalized_alpha * range)
+        }
+    }
 }