thread view: Inform when editing previous messages is unavailable (#36727)

Release Notes:

- N/A

crates/agent_ui/src/ui/unavailable_editing_tooltip.rs

@@ -0,0 +1,29 @@
+use gpui::{Context, IntoElement, Render, Window};
+use ui::{prelude::*, tooltip_container};
+
+pub struct UnavailableEditingTooltip {
+    agent_name: SharedString,
+}
+
+impl UnavailableEditingTooltip {
+    pub fn new(agent_name: SharedString) -> Self {
+        Self { agent_name }
+    }
+}
+
+impl Render for UnavailableEditingTooltip {
+    fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        tooltip_container(window, cx, |this, _, _| {
+            this.child(Label::new("Unavailable Editing")).child(
+                div().max_w_64().child(
+                    Label::new(format!(
+                        "Editing previous messages is not available for {} yet.",
+                        self.agent_name
+                    ))
+                    .size(LabelSize::Small)
+                    .color(Color::Muted),
+                ),
+            )
+        })
+    }
+}