Fix touch sensors under nested vmap

[sylvesterkaczmarek]

Fixes #3209.

Summary

Fix touch-sensor evaluation under nested jax.vmap by avoiding matrix multiplication in the contact-to-site coordinate transform. JAX's matmul batching rule can reorder nested batch dimensions in this path, producing an invalid dot_general shape.

The replacement uses equivalent elementwise products followed by a reduction over the vector dimension, preserving arbitrary leading batch axes.

A regression test uses the existing touch-sensor fixture under a 2 × 4 nested vmap and checks that the batched sensor output matches repeated unbatched evaluation.

Testing

• pytest mjx/mujoco/mjx/_src/sensor_test.py -q → 11 passed
• focused nested-vmap regression after minimizing the diff → 1 passed, 10 deselected
• pyink --check --fast mjx/mujoco/mjx/_src/sensor_test.py
• git diff --check