Fix Mermaid graph label quoting in README files

Updated the Mermaid diagrams in all README variants to quote the Wait (Backoff) node label, ensuring correct rendering of labels with parentheses.

README.zh-Hans.md

@@ -166,7 +166,7 @@ graph TD
     Upstream -->|成功| ProcessResponse[处理响应]
     Upstream -->|失败| Retry{重试?}
     
-    Retry -->|是| Wait[等待 (退避)] --> Upstream
+    Retry -->|是| Wait["等待 (退避)"] --> Upstream
     Retry -->|否| Error
     
     ProcessResponse --> Finalize[添加标头并返回]