fix(multiplayer): enforce drain at admission

Game/scripts/match_net.gd

@@ -169,9 +169,10 @@ static func reservation_identity_matches(slot_identity: String, incoming_identit
 func _on_peer_disconnected(peer_id: int) -> void:
 	if not multiplayer.is_server():
 		return
-	if not admissions_open:
-		await _reject(multiplayer.get_remote_sender_id(), "server is draining")
-		return
+	_cleanup_disconnected_peer(peer_id)
+
+
+func _cleanup_disconnected_peer(peer_id: int) -> void:
 	NetworkManager.invalidate_peer(peer_id)
 	_remove_player(peer_id)
 
@@ -210,7 +211,8 @@ func _remove_player(peer_id: int) -> void:
 	# from inside signal-handling: by then poll() has fully returned, every
 	# disconnect event in this batch has been dispatched, and get_peers()
 	# reflects the settled, genuinely-still-connected set.
-	call_deferred("_broadcast_player_left", peer_id)
+	if is_inside_tree():
+		call_deferred("_broadcast_player_left", peer_id)
 
 
 func _broadcast_player_left(peer_id: int) -> void:
@@ -239,6 +241,10 @@ static func _sanitize_shutdown_reason(raw: String) -> String:
 	return clean if not clean.is_empty() else "server_shutdown"
 
 
+static func admission_rejection(is_open: bool) -> String:
+	return "" if is_open else "server is draining"
+
+
 # Balances a new joiner onto whichever team currently has fewer players
 # (ties go to team 0). Server only.
 func _pick_balanced_team() -> int:
@@ -261,6 +267,10 @@ func _hello(protocol_version: int, tick_hz: int, player_name: String, supplied_j
 	var peer_id := multiplayer.get_remote_sender_id()
 	if roster.has(peer_id):
 		return   # duplicate hello from an already-accepted peer; ignore
+	var admission_error := admission_rejection(admissions_open)
+	if not admission_error.is_empty():
+		await _reject(peer_id, admission_error)
+		return
 
 	if protocol_version != NetCodec.PROTOCOL_VERSION:
 		await _reject(peer_id, "protocol version mismatch: server=%d client=%d" % [NetCodec.PROTOCOL_VERSION, protocol_version])