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/test/core/GdDiffToolTest.gd

@@ -0,0 +1,48 @@
+# GdUnit generated TestSuite
+#warning-ignore-all:unused_argument
+#warning-ignore-all:return_value_discarded
+class_name GdDiffToolTest
+extends GdUnitTestSuite
+
+# TestSuite generated from
+const __source = 'res://addons/gdUnit4/src/core/GdDiffTool.gd'
+
+
+func test_string_diff_empty() -> void:
+	var diffs := GdDiffTool.string_diff("", "")
+	assert_array(diffs).has_size(2)
+	assert_array(diffs[0]).is_empty()
+	assert_array(diffs[1]).is_empty()
+
+
+func test_string_diff_equals() -> void:
+	var diffs := GdDiffTool.string_diff("Abc", "Abc")
+	var expected_l_diff := "Abc".to_utf8_buffer()
+	var expected_r_diff := "Abc".to_utf8_buffer()
+
+	assert_array(diffs).has_size(2)
+	assert_array(diffs[0]).contains_exactly(expected_l_diff)
+	assert_array(diffs[1]).contains_exactly(expected_r_diff)
+
+
+func test_string_diff() -> void:
+	# tests the result of string diff function like assert_str("Abc").is_equal("abc")
+	var diffs := GdDiffTool.string_diff("Abc", "abc")
+	var chars := "Aabc".to_utf8_buffer()
+	var ord_A := chars[0]
+	var ord_a := chars[1]
+	var ord_b := chars[2]
+	var ord_c := chars[3]
+	var expected_l_diff := PackedByteArray([GdDiffTool.DIV_SUB, ord_A, GdDiffTool.DIV_ADD, ord_a, ord_b, ord_c])
+	var expected_r_diff := PackedByteArray([GdDiffTool.DIV_ADD, ord_A, GdDiffTool.DIV_SUB, ord_a, ord_b, ord_c])
+
+	assert_array(diffs).has_size(2)
+	assert_array(diffs[0]).contains_exactly(expected_l_diff)
+	assert_array(diffs[1]).contains_exactly(expected_r_diff)
+
+
+@warning_ignore("unused_parameter")
+func test_string_diff_large_value(fuzzer := Fuzzers.rand_str(1000, 4000), fuzzer_iterations := 10) -> void:
+	# test diff with large values not crashes the API GD-100
+	var value :String = fuzzer.next_value()
+	GdDiffTool.string_diff(value, value)