Fix typo in WebSocketClient::write buffer size clamp

Use sizeof(iTxBuffer) instead of sizeof(iTxSize) when computing the
remaining space, preventing truncated writes and potential memcpy overflow.

src/WebSocketClient.cpp

@@ -167,7 +167,7 @@ size_t WebSocketClient::write(const uint8_t *aBuffer, size_t aSize)
     // check if the write size, fits in the buffer
     if ((iTxSize + aSize) > sizeof(iTxBuffer))
     {
-        aSize = sizeof(iTxSize) - iTxSize;
+        aSize = sizeof(iTxBuffer) - iTxSize;
     }
 
     // copy data into the buffer