Ensure last XML is freed after tests.

This fixes undetected memory leaks in tests calling mj_loadXML.

Additionally, a specific test for `mj_freeLastXML` is introduced.

PiperOrigin-RevId: 764294041
Change-Id: I8b5a5683d8431ed920cdd52b06ad3515d8e19e14

test/fixture.h

@@ -53,6 +53,9 @@ class MujocoErrorTestGuard {
 // By default, any MuJoCo operation which triggers a warning or error will
 // trigger a test failure.
 class MujocoTest : public ::testing::Test {
+ public:
+  ~MujocoTest() { mj_freeLastXML(); }
+
  private:
   MujocoErrorTestGuard error_guard;
 };

test/xml/xml_api_test.cc

@@ -199,5 +199,15 @@ TEST_F(MujocoTest, SaveXmlWithDefaultMesh) {
   mj_deleteModel(saved_model);
 }
 
+TEST_F(MujocoTest, FreeLastXml) {
+  static constexpr char xml[] = "<mujoco/>";
+  mjModel* model = LoadModelFromString(xml, 0, 0);
+  ASSERT_THAT(model, NotNull());
+  mj_deleteModel(model);
+  ASSERT_NE(mj_saveLastXML(nullptr, nullptr, nullptr, 0), 0);
+  mj_freeLastXML();
+  ASSERT_EQ(mj_saveLastXML(nullptr, nullptr, nullptr, 0), 0);
+}
+
 }  // namespace
 }  // namespace mujoco