Unity 6.6 support

[sebas77]

The feature, motivation and pitch

Unity 6.6 deprecated GetInstanceId and now Only GetEntityId is available. However this test assumes something that is not true anymore

  [Test]
  public void SiblingRootsAreOrderedByInstanceId() {
    _fakeBodyA.enabled = true;
    _fakeBodyB.enabled = true;
    var mjcf = _scene.CreateScene(skipCompile:true);

#if UNITY_6000_5_OR_NEWER
    var first = EntityId.ToULong(_fakeBodyA.GetEntityId()) < EntityId.ToULong(_fakeBodyB.GetEntityId())
        ? _fakeBodyA : _fakeBodyB;
    var second = EntityId.ToULong(_fakeBodyA.GetEntityId()) < EntityId.ToULong(_fakeBodyB.GetEntityId())
        ? _fakeBodyB : _fakeBodyA;
#else
    var first = _fakeBodyA.GetInstanceID() < _fakeBodyB.GetInstanceID() ? _fakeBodyA : _fakeBodyB;
    var second = _fakeBodyA.GetInstanceID() < _fakeBodyB.GetInstanceID() ? _fakeBodyB : _fakeBodyA;
#endif

    int firstIndex = int.Parse(first.MujocoName.Replace("component_", ""));
    int secondIndex = int.Parse(second.MujocoName.Replace("component_", ""));
    Assert.That(firstIndex, Is.LessThan(secondIndex));

    var bodyNodes = mjcf.SelectNodes("/mujoco/worldbody/body");
    var bodyNames = new List<string>();
    foreach (XmlNode node in bodyNodes) {
      bodyNames.Add(node.Attributes["name"].Value);
    }
    Assert.That(bodyNames.IndexOf(first.MujocoName), Is.LessThan(bodyNames.IndexOf(second.MujocoName)));
  }

the new test I wrote cannot work, because the test was relying on the apparent monotonicity of GetInstanceId which is not a contract for the new GetEntityId. I couldn't open a PR because of this.

Alternatives

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Additional context

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