Fix bad message size returned from notification helpers

Author: moucha19-st

compost_rpc/compost_rpc.py

@@ -1702,7 +1702,7 @@ def generate(self, endpoint: Endpoint = Endpoint.REMOTE) -> list[tuple[Path, str
     size_t payload_len = dest - tx.payload_buf;
     tx.header.wlen = compost_bytes_to_words(payload_len);
     compost_header_store(tx_buf, tx.header);
-    return 4 + payload_len;
+    return COMPOST_MSG_LEN(tx_buf);
 }}
 
 """