fix(chat): handle multi-byte character truncation in tool result preview

libs/agent/chat/service.rs

@@ -791,7 +791,13 @@ impl ChatService {
                             crate::tool::ToolResult::Error(msg) => msg.clone(),
                         };
                         let preview = if text.len() > 300 {
-                            format!("{}...", &text[..300])
+                            let end = text
+                                .char_indices()
+                                .map(|(i, _)| i)
+                                .take_while(|&i| i <= 300)
+                                .last()
+                                .unwrap_or(300);
+                            format!("{}...", &text[..end])
                         } else {
                             text.clone()
                         };