Display what the tool is doing (#27120)

<img width="639" alt="Screenshot 2025-03-19 at 4 56 47 PM"
src="https://github.com/user-attachments/assets/b997f04d-4aff-4070-87b1-ffdb61019bd1"
/>

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>

crates/assistant_tools/src/thinking_tool.rs

@@ -31,6 +31,10 @@ impl Tool for ThinkingTool {
         serde_json::to_value(&schema).unwrap()
     }
 
+    fn ui_text(&self, _input: &serde_json::Value) -> String {
+        "Thinking".to_string()
+    }
+
     fn run(
         self: Arc<Self>,
         input: serde_json::Value,