fix(websocket): eliminate sign-conversion warning in payload handling

- iterate over char instead of unsigned char
- apply explicit cast to std::byte via unsigned char
- ensure safe and explicit byte conversion for binary frames

improves type safety and keeps websocket module warning-free

Author: GaspardKirira

src/session.cpp

@@ -573,9 +573,10 @@ namespace vix::websocket
         std::vector<std::byte> payload;
         payload.reserve(msg.data.size());
 
-        for (unsigned char ch : msg.data)
+        for (char ch : msg.data)
         {
-          payload.push_back(static_cast<std::byte>(ch));
+          payload.push_back(static_cast<std::byte>(
+              static_cast<unsigned char>(ch)));
         }
 
         frame = detail::build_frame(