test(capture): fixtures check client game messages, with a synthetic fixture

The fixture check now also reads every recorded client game message with the
struct the server reads it with (GameMessageHandler::CreateReceived) and writes
it again: it must read the whole message and give back the same bits. Game
message fields are decoded with the server's structs in the fixture tests.

A synthetic fixture, built in the test from the server's own structs (login,
position update, a client game message), goes through the export steps
(portable, anonymised, saved, read again) and passes the same checks; recorded
fixtures stay in tests/fixtures-local and are never committed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dGame/dGameMessages/GameMessageHandler.cpp

@@ -269,3 +269,8 @@ void GameMessageHandler::HandleMessage(RakNet::BitStream& inStream, const System
 
 	LOG_DEBUG("Received Unknown GM with ID: %4i, %s", messageID, StringifiedEnum::ToString(messageID).data());
 }
+
+std::unique_ptr<GameMessages::NetGameMsg> GameMessageHandler::CreateReceived(MessageType::Game messageID) {
+	const auto handler = g_MessageHandlers.find(messageID);
+	return handler == g_MessageHandlers.end() ? nullptr : handler->second();
+}