v36tov35: add tests for special mode bits masking

Add tests to verify that special mode bits (setuid/setgid/sticky)
are correctly masked out when down-translating from v3.6 to v3.5.

v3.6 supports special mode bits, but v3.5 and earlier do not, so
the bits must be masked during translation to avoid applying
permissions that would not have been applied in earlier versions.

Author: prestist

translate_test.go

@@ -3295,6 +3295,56 @@ func TestTranslate3_6to3_5(t *testing.T) {
 	}
 	assert.Equal(t, downtranslateConfig3_5, res)
 
+	// Test that special mode bits are correctly masked out during translation
+	res, err = v36tov35.Translate(types3_6.Config{
+		Ignition: types3_6.Ignition{
+			Version: "3.6.0",
+		},
+		Storage: types3_6.Storage{
+			Files: []types3_6.File{
+				{
+					Node: types3_6.Node{
+						Path: "/root/file.txt",
+					},
+					FileEmbedded1: types3_6.FileEmbedded1{
+						Mode: util.IntP(01777),
+					},
+				},
+			},
+		},
+	})
+	assert.NoError(t, err)
+	// Verify that special mode bits were masked out (01777 -> 0777)
+	assert.Equal(t, util.IntP(0777), res.Storage.Files[0].Mode)
+
+	res, err = v36tov35.Translate(types3_6.Config{
+		Ignition: types3_6.Ignition{
+			Version: "3.6.0",
+		},
+		Storage: types3_6.Storage{
+			Directories: []types3_6.Directory{
+				{
+					Node: types3_6.Node{
+						Path:      "/rootdir",
+						Overwrite: util.BoolP(true),
+						User: types3_6.NodeUser{
+							ID: util.IntP(1000),
+						},
+						Group: types3_6.NodeGroup{
+							Name: util.StrP("groupname"),
+						},
+					},
+					DirectoryEmbedded1: types3_6.DirectoryEmbedded1{
+						Mode: util.IntP(01777),
+					},
+				},
+			},
+		},
+	})
+	assert.NoError(t, err)
+	// Verify that special mode bits were masked out (01777 -> 0777)
+	assert.Equal(t, util.IntP(0777), res.Storage.Directories[0].Mode)
+
 	// Translation with unsupported custom clevis pin
 	_, err = v36tov35.Translate(types3_6.Config{
 		Ignition: types3_6.Ignition{