assistant2: Factor out tool use into its own module (#25819)

This PR factors out the concerns related to tool use out of `Thread` and
into their own module.

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_use;
 mod ui;
 
 use std::sync::Arc;