Move CompareModel to a separate library.

PiperOrigin-RevId: 927174846
Change-Id: Ia8b8b2c8593133d8910399be755e614282bf3252

test/compare_model.h

@@ -0,0 +1,30 @@
+// 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.
+
+#ifndef MUJOCO_TEST_COMPARE_MODEL_H_
+#define MUJOCO_TEST_COMPARE_MODEL_H_
+
+#include <string>
+
+#include <mujoco/mujoco.h>
+
+namespace mujoco {
+
+// Compares all fields of two mjModels.
+// Returns the name of the different field and the max difference.
+mjtNum CompareModel(const mjModel* m1, const mjModel* m2, std::string& field);
+
+}  // namespace mujoco
+
+#endif  // MUJOCO_TEST_COMPARE_MODEL_H_