fix(multiplayer): validate ticket playlists

Game/scripts/matchmaking_state.gd

@@ -51,6 +51,8 @@ func begin_queue(new_ticket_id: String, new_playlist: String) -> bool:
 func apply_ticket_update(update: Dictionary) -> bool:
 	if not _has_string(update, "ticket_id") or not update.has("revision") or not _valid_revision(update["revision"]) or not update.has("state"):
 		return _request_resync(self.ticket_id)
+	if update.has("playlist") and not _valid_playlist(String(update["playlist"])):
+		return _request_resync(self.ticket_id)
 	if ticket_id.is_empty() or String(update["ticket_id"]) != ticket_id:
 		return _request_resync(self.ticket_id)
 	var incoming_revision := int(update["revision"])
@@ -299,3 +301,7 @@ func _valid_revision(value: Variant) -> bool:
 	if value is float:
 		return is_finite(float(value)) and float(value) >= 0.0 and float(value) == floor(float(value))
 	return false
+
+
+func _valid_playlist(value: String) -> bool:
+	return value == "casual" or value == "ranked"

Game/tests/cases/test_matchmaking_state.gd

@@ -82,6 +82,14 @@ func test_ticket_and_proposal_revisions_must_be_nonnegative_integers() -> void:
 	assert_true(not state.apply_proposal_update({"proposal_id": "proposal-revision", "revision": -1, "state": "OPEN"}), "negative proposal revision is rejected")
 
 
+func test_ticket_update_rejects_invalid_playlist_without_mutation() -> void:
+	var state := MatchmakingState.new()
+	state.begin_queue("ticket-playlist", "casual")
+	assert_true(not state.apply_ticket_update({"ticket_id": "ticket-playlist", "revision": 1, "state": "PROPOSED", "playlist": "admin"}), "unknown playlist is rejected")
+	assert_eq(state.phase, MatchmakingState.QUEUED, "invalid playlist cannot change phase")
+	assert_eq(state.playlist, "casual", "invalid playlist cannot change playlist")
+
+
 func test_proposal_terminal_states_are_visible_and_not_cancellable() -> void:
 	var state := MatchmakingState.new()
 	state.begin_queue("ticket-1", "casual")