feat: 添加文件读取工具的字符截断逻辑，记录截断字符数量

src/tools/file_read.rs

@@ -179,6 +179,7 @@ impl Tool for FileReadTool {
 
                 // Truncate if too long
                 if result.len() > MAX_CHARS {
+                    let original_len = result.len();
                     let mut truncated_chars = 0;
                     let mut end_idx = 0;
                     for (i, line) in lines.iter().enumerate() {
@@ -190,9 +191,10 @@ impl Tool for FileReadTool {
                         end_idx = i + 1;
                     }
                     result = lines[..end_idx].join("\n");
+                    let truncated_amount = original_len - result.len();
                     result.push_str(&format!(
                         "\n\n... ({} chars truncated) ...",
-                        result.len() - MAX_CHARS
+                        truncated_amount
                     ));
                 }