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/GameMessageDecoder.h

@@ -19,6 +19,13 @@ namespace GameMessageDecoder {
 	// The message's fields, read from `payload` (the bits after the object ID and message ID), or nullopt when it has
 	// no typed struct or doesn't read cleanly
 	std::optional<nlohmann::json> Decode(MessageType::Game messageId, bool toServer, RakNet::BitStream& payload);
+
+	/**
+	 * Reads a message a client sent with the struct the server reads it with, and writes it again. nullopt when the
+	 * server has no typed struct for it or it doesn't read; otherwise whether it read the whole message (padding
+	 * after the last field aside) and the same bits came back. The capture fixture tests run it on every recorded client message.
+	 */
+	std::optional<bool> RoundTripReceived(MessageType::Game messageId, RakNet::BitStream& payload);
 }
 
 #endif  //!__GAMEMESSAGEDECODER__H__