Restore tool cards on thread deserialization (#30053)

Release Notes:

- N/A

---------

Co-authored-by: Julia Ryan <juliaryan3.14@gmail.com>

crates/assistant_tools/src/fetch_tool.rs

@@ -166,7 +166,7 @@ impl Tool for FetchTool {
                     bail!("no textual content found");
                 }
 
-                Ok(text)
+                Ok(text.into())
             })
             .into()
     }