MJX plane-capsule collision numerical safety

### Intro

MJX uses a [rather aggressive criterion](https://github.com/google-deepmind/mujoco/blob/9e7979e4925b931e835508d47078e8f199ac93ec/mjx/mujoco/mjx/_src/collision_primitive.py#L75-L76) in the plane-capsule collision to prevent numerical issues (I think?), leading to some (seemingly) minor numerical differences with MuJoCo, which doesn't seem to implement this check. So not a bug per se, but it does lead to unexpected results when comparing to mujoco.

It would be great if we could adjust the strength here, or some flag to enable/disable. Happy to make a PR if you're interested.