Fix focus handle leak (#26090)

This fixes a major performance issue in the current git beta.
This PR also removes the PopoverButton component, which was easy to
misuse.

Release Notes:

- Git Beta: Fix frame drops caused by opening the git panel

---------

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/assistant2/src/message_editor.rs

@@ -8,6 +8,7 @@ use gpui::{
     TextStyle, WeakEntity,
 };
 use language_model::LanguageModelRegistry;
+use language_model_selector::ToggleModelSelector;
 use rope::Point;
 use settings::Settings;
 use std::time::Duration;
@@ -297,11 +298,9 @@ impl Render for MessageEditor {
         v_flex()
             .key_context("MessageEditor")
             .on_action(cx.listener(Self::chat))
-            .on_action(cx.listener(|this, action, window, cx| {
-                let selector = this.model_selector.read(cx).selector.clone();
-                selector.update(cx, |this, cx| {
-                    this.toggle_model_selector(action, window, cx);
-                })
+            .on_action(cx.listener(|this, _: &ToggleModelSelector, window, cx| {
+                this.model_selector
+                    .update(cx, |model_selector, cx| model_selector.toggle(window, cx));
             }))
             .on_action(cx.listener(Self::toggle_context_picker))
             .on_action(cx.listener(Self::remove_all_context))