Fix clippy derivable_impls warning

Replace manual Default impl for QwenErrorResult with #[derive(Default)]
as suggested by clippy. This is equivalent since all fields have default
values (bool defaults to false, Option defaults to None).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: konard

rust/src/tools/qwen.rs

@@ -268,23 +268,13 @@ pub fn extract_usage(output: &str) -> QwenUsage {
 }
 
 /// Error detection result
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Default)]
 pub struct QwenErrorResult {
     pub has_error: bool,
     pub error_type: Option<String>,
     pub message: Option<String>,
 }
 
-impl Default for QwenErrorResult {
-    fn default() -> Self {
-        Self {
-            has_error: false,
-            error_type: None,
-            message: None,
-        }
-    }
-}
-
 /// Detect errors in Qwen Code output
 ///
 /// # Arguments