agent_ui: Display footer for model selector when in Zed agent (#43294)

This PR adds back the footer with the "Configure" button in the model
selector but only when the seeing it from the Zed agent (or inline
assistant/text threads). I had removed it a while back because seeing
the "Configure" button, which takes you to the agent panel settings
view, when clicking from an external agent didn't make much sense, given
there's nothing model-wise you can configure from Zed (at least yet) for
an external agent.

This also makes the button in the footer a bit nicer by making it full
screen and displaying a keybinding, so that you can easily do the whole
"trigger model selector → go to settings view" all with the keyboard.

<img width="400" height="870" alt="Screenshot 2025-11-21 at 10  38@2x"
src="https://github.com/user-attachments/assets/c14f2acf-b793-4bc1-ac53-8a8a53b219e6"
/>

Release Notes:

- N/A

crates/agent/src/agent.rs

@@ -961,6 +961,10 @@ impl acp_thread::AgentModelSelector for NativeAgentModelSelector {
     fn watch(&self, cx: &mut App) -> Option<watch::Receiver<()>> {
         Some(self.connection.0.read(cx).models.watch())
     }
+
+    fn should_render_footer(&self) -> bool {
+        true
+    }
 }
 
 impl acp_thread::AgentConnection for NativeAgentConnection {