Unpin agent thread controls (#35661)

This PR moves the new agent thread controls so they're attached to the
last message and scroll with the thread history, instead of always being
shown above the message editor.

Release Notes:

- N/A

crates/agent_ui/src/acp/thread_view.rs

@@ -785,7 +785,7 @@ impl AcpThreadView {
         window: &mut Window,
         cx: &Context<Self>,
     ) -> AnyElement {
-        match &entry {
+        let primary = match &entry {
             AgentThreadEntry::UserMessage(message) => div()
                 .py_4()
                 .px_2()
@@ -850,6 +850,19 @@ impl AcpThreadView {
                 .px_5()
                 .child(self.render_tool_call(index, tool_call, window, cx))
                 .into_any(),
+        };
+
+        let Some(thread) = self.thread() else {
+            return primary;
+        };
+        let is_generating = matches!(thread.read(cx).status(), ThreadStatus::Generating);
+        if index == total_entries - 1 && !is_generating {
+            v_flex()
+                .child(primary)
+                .child(self.render_thread_controls(cx))
+                .into_any_element()
+        } else {
+            primary
         }
     }
 
@@ -2404,7 +2417,7 @@ impl AcpThreadView {
         }
     }
 
-    fn render_thread_controls(&mut self, cx: &mut Context<Self>) -> impl IntoElement {
+    fn render_thread_controls(&self, cx: &Context<Self>) -> impl IntoElement {
         let open_as_markdown = IconButton::new("open-as-markdown", IconName::FileText)
             .icon_size(IconSize::XSmall)
             .icon_color(Color::Ignored)
@@ -2425,9 +2438,8 @@ impl AcpThreadView {
             }));
 
         h_flex()
-            .mt_1()
             .mr_1()
-            .py_2()
+            .pb_2()
             .px(RESPONSE_PADDING_X)
             .opacity(0.4)
             .hover(|style| style.opacity(1.))
@@ -2487,18 +2499,12 @@ impl Render for AcpThreadView {
 
                     v_flex().flex_1().map(|this| {
                         if self.list_state.item_count() > 0 {
-                            let is_generating =
-                                matches!(thread_clone.read(cx).status(), ThreadStatus::Generating);
-
                             this.child(
                                 list(self.list_state.clone())
                                     .with_sizing_behavior(gpui::ListSizingBehavior::Auto)
                                     .flex_grow()
                                     .into_any(),
                             )
-                            .when(!is_generating, |this| {
-                                this.child(self.render_thread_controls(cx))
-                            })
                             .children(match thread_clone.read(cx).status() {
                                 ThreadStatus::Idle | ThreadStatus::WaitingForToolConfirmation => {
                                     None