chore(client): add gdUnit4 test framework addon

Vendor gdUnit4 for Godot 4 client testing (D-030).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/addons/gdUnit4/src/network/rpc/RPCClientConnect.gd

@@ -0,0 +1,13 @@
+class_name RPCClientConnect
+extends RPC
+
+var _client_id: int
+
+
+func with_id(id: int) -> RPCClientConnect:
+	_client_id = id
+	return self
+
+
+func client_id() -> int:
+	return _client_id