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/AsyncQueue.hpp

@@ -41,6 +41,8 @@
 #include <exception>
 #include <functional>
 #include <mutex>
+#include <stdexcept>
+#include <string>
 #include <thread>
 #include <utility>
 #include <vector>
@@ -87,7 +89,20 @@ public:
           _running(false),
           _stop(false),
           _wakeup(false),
-          _dropped(0) {}
+          _dropped(0) {
+        if (!_handler) {
+            throw std::invalid_argument("AsyncQueue: handler must not be null");
+        }
+        if (config.batch_size == 0) {
+            throw std::invalid_argument("AsyncQueue: batch_size must be > 0");
+        }
+        if (config.flush_interval <= 0 || config.flush_interval > 300.0) {
+            throw std::invalid_argument("AsyncQueue: flush_interval must be in (0, 300] seconds");
+        }
+        if (config.poll_timeout <= 0 || config.poll_timeout > 5.0) {
+            throw std::invalid_argument("AsyncQueue: poll_timeout must be in (0, 5] seconds");
+        }
+    }
 
     /// 析构函数：自动 shutdown()，保证队列中剩余数据被 flush。
     ~AsyncQueue() { shutdown(); }