Add copyright notice to test protos

PiperOrigin-RevId: 630517509

conformance/test_protos/test_messages_edition2023.proto

@@ -1,3 +1,10 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2024 Google Inc.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
 edition = "2023";
 
 package protobuf_test_messages.editions;