Fix Response.data normalisation in parse_message() too

The previous commit only fixed Response.from_json(), but the serial
client's read loop uses parse_message() which constructs Response
directly. Apply the same string-to-dict normalisation there.

src/mcbluetooth_esp32/protocol.py

@@ -207,11 +207,14 @@ def parse_message(line: str) -> Command | Response | Event:
         )
 
     if msg_type == MsgType.RESP:
+        raw_data = obj.get("data", {})
+        if isinstance(raw_data, str):
+            raw_data = {"error": raw_data}
         return Response(
             type=MsgType.RESP,
             id=obj["id"],
             status=Status(obj["status"]),
-            data=obj.get("data", {}),
+            data=raw_data if isinstance(raw_data, dict) else {},
         )
 
     if msg_type == MsgType.EVENT: