assistant2: Add tool selector (#26480)

This PR adds a tool selector to Assistant 2 to facilitate customizing
the tools that the model sees:

<img width="1297" alt="Screenshot 2025-03-11 at 4 25 31 PM"
src="https://github.com/user-attachments/assets/7a656343-83bc-4546-9430-6a5f7ff1fd08"
/>

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -16,6 +16,7 @@ mod terminal_inline_assistant;
 mod thread;
 mod thread_history;
 mod thread_store;
+mod tool_selector;
 mod tool_use;
 mod ui;