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/cmd/CmdCommand.gd

@@ -0,0 +1,27 @@
+class_name CmdCommand
+extends RefCounted
+
+var _name: String
+var _arguments: PackedStringArray
+
+
+func _init(p_name :String, p_arguments := []) -> void:
+	_name = p_name
+	_arguments = PackedStringArray(p_arguments)
+
+
+func name() -> String:
+	return _name
+
+
+func arguments() -> PackedStringArray:
+	return _arguments
+
+
+func add_argument(arg :String) -> void:
+	@warning_ignore("return_value_discarded")
+	_arguments.append(arg)
+
+
+func _to_string() -> String:
+	return "%s:%s" % [_name, ", ".join(_arguments)]