MJX: Enable `impl="warp"` on MacOS

[hartikainen]

The feature, motivation and pitch

It would be nice to enable the mjx's impl="warp" on MacOS. Most of our developers use MacOS for their day-to-day development and it would be really convenient to debug warp models locally.

This wasn't possible previously because warp_lang itself didn't provide MacOS wheels, but it seems like they've been available at least since warp-lang==1.9.0.dev20250731. Perhaps there's some other caveats that I'm not aware of that make this hard.

This is a bit forward-looking request and not a super urgent/high-priority, but if it's a low-hanging fruit, it might be worth considering.

Alternatives

• Use impl="jax" to debug on MacOS. Not a great solution because there are already features that are supported by impl="warp" but not by impl="jax".
• Debug on Linux machine remotely.

Additional context

I put up a simple proof of concept in #2948. The {Model,Data}IOTest in io_test.py at least pass without problems when just removing the warp cuda checks.