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/regex_search_tool.rs

@@ -22,10 +22,17 @@ pub struct RegexSearchToolInput {
     /// Optional starting position for paginated results (0-based).
     /// When not provided, starts from the beginning.
     #[serde(default)]
-    pub offset: Option<usize>,
+    pub offset: Option<u32>,
 }
 
-const RESULTS_PER_PAGE: usize = 20;
+impl RegexSearchToolInput {
+    /// Which page of search results this is.
+    pub fn page(&self) -> u32 {
+        1 + (self.offset.unwrap_or(0) / RESULTS_PER_PAGE)
+    }
+}
+
+const RESULTS_PER_PAGE: u32 = 20;
 
 pub struct RegexSearchTool;
 
@@ -43,6 +50,24 @@ impl Tool for RegexSearchTool {
         serde_json::to_value(&schema).unwrap()
     }
 
+    fn ui_text(&self, input: &serde_json::Value) -> String {
+        match serde_json::from_value::<RegexSearchToolInput>(input.clone()) {
+            Ok(input) => {
+                let page = input.page();
+
+                if page > 1 {
+                    format!(
+                        "Get page {page} of search results for regex “`{}`”",
+                        input.regex
+                    )
+                } else {
+                    format!("Search files for regex “`{}`”", input.regex)
+                }
+            }
+            Err(_) => "Search with regex".to_string(),
+        }
+    }
+
     fn run(
         self: Arc<Self>,
         input: serde_json::Value,
@@ -154,7 +179,7 @@ impl Tool for RegexSearchTool {
                     offset + matches_found,
                     offset + RESULTS_PER_PAGE,
                 ))
-          } else {
+            } else {
                 Ok(format!("Found {matches_found} matches:\n{output}"))
             }
         })