Accept NumPy integers for rollout sizing

[sylvesterkaczmarek]

Summary

Allow rollout to accept NumPy integer scalar values for nstep and chunk_size.

These values commonly come from NumPy computations. np.int64(3) is a valid integral value but is not an instance of Python's built-in int, so current validation rejects it before the rollout reaches the native implementation.

This change:

• accepts np.integer alongside Python int for nstep
• accepts np.integer alongside Python int for chunk_size
• adds focused regression coverage for both arguments
• keeps all existing dimension and value checks unchanged

This revives the still-relevant fix from closed, unmerged PR #3300 against current main.

Validation

The final branch is based directly on current main and contains one focused commit changing only python/mujoco/rollout.py plus a regression test.

The MuJoCo Python extension was not built in this execution environment, so the new runtime regression test was not executed locally and no local test pass is claimed.