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/RpcClient.hpp

@@ -74,7 +74,14 @@ public:
         : _addr(addr), _port(port), _name(name),
           _timeout(timeout),
           _retry_on_refused(retry_on_refused),
-          _retry_interval(retry_interval) {}
+          _retry_interval(retry_interval) {
+        if (timeout <= 0 || timeout > 300.0) {
+            throw MsgException("invalid timeout: " + std::to_string(timeout));
+        }
+        if (retry_interval <= 0 || retry_interval > 60.0) {
+            throw MsgException("invalid retry_interval: " + std::to_string(retry_interval));
+        }
+    }
 
     // ---- RPC 调用 ----------------------------------------------------------