chore: print per-source reason chain on soak call failure

tools/soak/run_soak.py

@@ -158,7 +158,11 @@ async def _worker_async(args: argparse.Namespace, worker_idx: int) -> None:
             raise
         except Exception as exc:  # 记分板从遥测读错误分布;这里只计数
             stats["failed"] += 1
-            print(f"[w{worker_idx}] 调用失败: {type(exc).__name__}: {exc}", flush=True)
+            chain = getattr(exc, "per_source_reasons", None) or {}
+            print(
+                f"[w{worker_idx}] 调用失败: {type(exc).__name__}: {exc} 链={chain}",
+                flush=True,
+            )
 
     def _should_stop() -> bool:
         return (