Fix #3368: clip hfield grid indices before int cast in MJX collision

[Ashutosh0x]

Problem

In _hfield_collision, the computed grid indices cmin and rmin are float values that may be very large (e.g. +inf or beyond int32 range) when the colliding object is far outside the height field. Calling .astype(int) directly on such values produces an integer overflow, which generates a JAX/XLA warning and results in undefined behaviour on GPU targets.

Note: PR #3369 fixes a related overflow in the _box_box and _convex_convex paths. This PR addresses the remaining overflow in the hfield path.

Fixes #3368.

Changes

mjx/mujoco/mjx/_src/collision_convex.py

Clip cmin and rmin to a valid range before calling .astype(int):

# Before
cmin = jp.floor(...).astype(int)
rmin = jp.floor(...).astype(int)

# After
cmin = jp.clip(jp.floor(...), -(h.ncol - 1), h.ncol - 1).astype(int)
rmin = jp.clip(jp.floor(...), -(h.nrow - 1), h.nrow - 1).astype(int)

The clip bounds match the existing clamps inside make_prisms (jp.clip(ri, 0, h.nrow - 2) / jp.clip(ci, 0, h.ncol - 2)), so the observable collision result is unchanged for in-bounds objects. For out-of-bounds objects the prisms simply land on the nearest valid row/column, which is the correct fallback behaviour.