Inline assistant v2 layout (#22305)

Makes the inline assistant look like @danilo-leal's prototype.

Also fixes the bug with indent guides that @maxdeviant found!

<img width="1059" alt="Screenshot 2024-12-20 at 4 24 56 PM"
src="https://github.com/user-attachments/assets/5e55a3c2-768a-4e9d-bad5-d4ebbe9db9ce"
/>

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -1,4 +1,5 @@
 mod active_thread;
+mod assistant_model_selector;
 mod assistant_panel;
 mod assistant_settings;
 mod buffer_codegen;