fix(multiplayer): enforce proposal transitions

Game/scripts/matchmaking_state.gd

@@ -112,15 +112,28 @@ func apply_proposal_update(update: Dictionary) -> bool:
 		return _request_resync(proposal_id)
 	var incoming_proposal_state := String(update["state"])
 	if incoming_proposal_state == "OPEN":
+		if not _is_legal_proposal_transition(proposal_state, incoming_proposal_state):
+			return _request_resync(proposal_id)
 		phase = PROPOSED
 	elif incoming_proposal_state == "ACCEPTED":
+		if not _is_legal_proposal_transition(proposal_state, incoming_proposal_state):
+			return _request_resync(proposal_id)
 		phase = ALLOCATING
 	elif incoming_proposal_state == "DECLINED":
+		if not _is_legal_proposal_transition(proposal_state, incoming_proposal_state):
+			return _request_resync(proposal_id)
 		phase = FAILED
 		message = "A player declined the match proposal"
 	elif incoming_proposal_state == "EXPIRED":
+		if not _is_legal_proposal_transition(proposal_state, incoming_proposal_state):
+			return _request_resync(proposal_id)
 		phase = EXPIRED
 		message = "The match proposal expired"
+	elif incoming_proposal_state == "CANCELLED":
+		if not _is_legal_proposal_transition(proposal_state, incoming_proposal_state):
+			return _request_resync(proposal_id)
+		phase = CANCELLED
+		message = "The match proposal was cancelled"
 	else:
 		return _request_resync(proposal_id)
 	proposal_revision = incoming_revision
@@ -268,6 +281,14 @@ func _is_legal_ticket_transition(from: String, to: String) -> bool:
 	return transitions.has(from) and to in transitions[from]
 
 
+func _is_legal_proposal_transition(from: String, to: String) -> bool:
+	if from.is_empty():
+		return to == "OPEN"
+	if from == to:
+		return true
+	return from == "OPEN" and to in ["ACCEPTED", "DECLINED", "EXPIRED", "CANCELLED"]
+
+
 func _has_string(value: Dictionary, key: String) -> bool:
 	return value.has(key) and value[key] is String and not String(value[key]).is_empty()