assistant2: Change model selector keybinding and make it visible (#22965)

We weren't showing the keybinding in none of the places where the model
selector was visible. Also, I took advantage of the opportunity to
change the keybinding for two reasons:

1. `cmd-shift-m` caused conflict if on an editor (inline assistant case)
2. `cmd-opt-/` is the one Cursor uses; so consistency with something
that might be already consolidated sounds like a low-hanging fruit

| Editor Inline Assist | Terminal Inline Assist | Assistant Panel |
|--------|--------|--------|
| <img width="1336" alt="Screenshot 2025-01-10 at 11 01 24 AM"
src="https://github.com/user-attachments/assets/0782f217-025f-4bc0-b2fa-64b3524c968b"
/> | <img width="1336" alt="Screenshot 2025-01-10 at 11 01 29 AM"
src="https://github.com/user-attachments/assets/d05a3b5c-33fd-4593-b1d8-aa9944de816a"
/> | <img width="1336" alt="Screenshot 2025-01-10 at 11 01 33 AM"
src="https://github.com/user-attachments/assets/8cb075e7-ccde-46f5-aa05-d20a9d42b286"
/> |

Release Notes:

- N/A

crates/assistant2/src/message_editor.rs

@@ -100,7 +100,12 @@ impl MessageEditor {
             inline_context_picker,
             inline_context_picker_menu_handle,
             model_selector: cx.new_view(|cx| {
-                AssistantModelSelector::new(fs, model_selector_menu_handle.clone(), cx)
+                AssistantModelSelector::new(
+                    fs,
+                    model_selector_menu_handle.clone(),
+                    editor.focus_handle(cx),
+                    cx,
+                )
             }),
             model_selector_menu_handle,
             use_tools: false,