fix(libmsg): complete security audit round 2 - fix P0 race + harden input validation

- P0: fix _close_all_clients() race condition (CWE-362) - hold per-connection send_lock
  before close() to prevent messages being sent to wrong client after fd reuse
- P2: add comprehensive input validation for all public APIs
  (timeout/port/workers/max_connections/batch_size etc. bounds checking)
- P2: add UTF-8 payload validation (isValidUtf8) aligning with Python pymsg behavior,
  rejecting overlong encodings, surrogate pairs, truncated/invalid sequences
- P2: document pmsg_recv(int) precondition and recommend deadline mode
- add constructor validation for MsgClient/MsgServer/AsyncQueue/RpcClient/RpcServer
- add add_route() validation for empty route name and null handler
- update security audit report with round 2 findings and fixes (8 issues total, 100% fixed)

libmsg/inc/us/RpcServer.hpp

@@ -80,10 +80,16 @@ public:
 
     /** 注册路由处理函数。
      *
-     * @param name     路由名（对应请求中的 _route 字段）
+     * @param name     路由名（对应请求中的 _route 字段），不能为空
      * @param handler  处理函数，签名 json::value(const json::object& req)
      */
     void add_route(const std::string& name, Handler handler) {
+        if (name.empty()) {
+            throw MsgException("route name cannot be empty");
+        }
+        if (!handler) {
+            throw MsgException("route handler cannot be null");
+        }
         _routes[name] = std::move(handler);
     }