fix: resolve mypy type errors in AsyncRealtimeClient

- Add class-level type annotation for serializer: Optional[Serializer]
- Add type annotation for message_data: Union[str, bytes]
- Fix type compatibility issues with websocket send method

Author: grdsdev

src/realtime/src/realtime/_async/client.py

@@ -42,6 +42,7 @@ def wrapper(*args, **kwargs):
 
 
 class AsyncRealtimeClient:
+    serializer: Optional[Serializer]
     def __init__(
         self,
         url: str,
@@ -375,6 +376,7 @@ async def send(self, message: Union[Message, Dict[str, Any]]) -> None:
             msg = Message(**message)
 
         # Encode message based on serializer version
+        message_data: Union[str, bytes]
         if self.vsn == VSN_2_0_0 and self.serializer:
             # Convert Message to dict for serializer
             msg_dict = {