feat: add TypedMessageHandler + generic Decode[T] helper

- handler: add OnTypedMessage() for typed NATS message callbacks
  Avoids double-decode (msgpack→map→typed) by skipping map step
- handler: refactor wrapHandler into wrapTypedHandler + wrapMapHandler
- natsutil: add generic Decode[T](data) for direct msgpack→struct decode
- tests: add typed handler tests + benchmark (11 tests pass)

natsutil/natsutil.go

@@ -124,6 +124,14 @@ func DecodeMsgpack(msg *nats.Msg, dest any) error {
 	return msgpack.Unmarshal(msg.Data, dest)
 }
 
+// Decode is a generic helper that unmarshals msgpack bytes into T.
+// Usage:  req, err := natsutil.Decode[messages.ChatRequest](msg.Data)
+func Decode[T any](data []byte) (T, error) {
+	var v T
+	err := msgpack.Unmarshal(data, &v)
+	return v, err
+}
+
 // DecodeMsgpackMap decodes msgpack data into a generic map.
 func DecodeMsgpackMap(data []byte) (map[string]any, error) {
 	var m map[string]any