assistant2: Avoid unnecessary String cloning in tool use (#25725)

This PR removes some unnecessary `String` cloning in the tool use paths.

We now store the data in `Arc<str>`s for cheap cloning.

Release Notes:

- N/A

crates/language_model/src/language_model.rs

@@ -90,7 +90,7 @@ where
 #[derive(Debug, PartialEq, Eq, Hash, Clone, Serialize, Deserialize)]
 pub struct LanguageModelToolUse {
     pub id: LanguageModelToolUseId,
-    pub name: String,
+    pub name: Arc<str>,
     pub input: serde_json::Value,
 }