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_ui/src/text_thread_editor.rs

@@ -280,6 +280,8 @@ impl TextThreadEditor {
             .thought_process_output_sections()
             .to_vec();
         let slash_commands = text_thread.read(cx).slash_commands().clone();
+        let focus_handle = editor.read(cx).focus_handle(cx);
+
         let mut this = Self {
             text_thread,
             slash_commands,
@@ -315,6 +317,7 @@ impl TextThreadEditor {
                         });
                     },
                     true, // Use popover styles for picker
+                    focus_handle,
                     window,
                     cx,
                 )