Move mjz_decoder out of experimental to xml/mjz.

Since the mjz format is just an archive for MJCF assets this seems like a sensible place. The upcoming mjz encoder will also need to make use of the full precision XML utility that is local to src/xml.

PiperOrigin-RevId: 912044746
Change-Id: I6a9fef24b1c3fec5b5edc3a8d4c8e22584273264

test/xml/mjz/CMakeLists.txt

@@ -0,0 +1,19 @@
+# Copyright 2021 DeepMind Technologies Limited
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+mujoco_test(mjz_api_test)
+
+mujoco_test(
+  mjz_decoder_test
+)

test/xml/mjz/mjz_decoder_test.cc

@@ -0,0 +1,60 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <string>
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+#include <mujoco/mjspec.h>
+#include <mujoco/mujoco.h>
+#include "test/fixture.h"
+
+namespace mujoco {
+namespace {
+
+using MjzTest = MujocoTest;
+
+using ::testing::IsNull;
+using ::testing::Not;
+using ::testing::NotNull;
+using ::testing::StrEq;
+
+TEST_F(MjzTest, Parse) {
+  mjVFS vfs;
+  mj_defaultVFS(&vfs);
+  std::string filepath = GetTestDataFilePath("testdata/model.mjz");
+  char err[1000] = "";
+  mjSpec* spec = mj_parse(filepath.c_str(), "", &vfs, err, sizeof(err));
+  EXPECT_THAT(spec, NotNull());
+  EXPECT_THAT(err, StrEq(""));
+  mjModel* model = mj_compile(spec, &vfs);
+  EXPECT_THAT(model, NotNull()) << mjs_getError(spec);
+  mj_deleteModel(model);
+  mj_deleteSpec(spec);
+  mj_deleteVFS(&vfs);
+}
+
+TEST_F(MjzTest, InvalidPath) {
+  mjVFS vfs;
+  mj_defaultVFS(&vfs);
+  std::string filepath = GetTestDataFilePath("testdata/not_a_file.mjz");
+  char err[1000] = "";
+  mjSpec* spec = mj_parse(filepath.c_str(), "", nullptr, err, sizeof(err));
+  EXPECT_THAT(spec, IsNull());
+  EXPECT_THAT(err, Not(StrEq("")));
+  mj_deleteVFS(&vfs);
+}
+
+}  // namespace
+}  // namespace mujoco