Fix RPC path mismatch in headless server

The headless server added the multiplayer-fps game scene as a child node
(/root/HeadlessServer/multiplayer-fps/...), causing RPC node paths to
include the HeadlessServer/ prefix. Clients expected paths relative to
root (/root/multiplayer-fps/...), so every RPC would fail with
'Node not found'.

Fix: add the game scene directly to /root/ via call_deferred, making it
a sibling of the HeadlessServer node rather than a child. RPC paths now
match between server and client.

test_connect.tscn

@@ -0,0 +1,6 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="res://test_connect.gd" id="1_script"]
+
+[node name="TestConnect" type="Node"]
+script = ExtResource("1_script")