[MJCF] Improper solref setting when using unit scaling

[ShravanTata]

Description

The solref parameter is being modified directly when passed to a function due to the parameter being a list, which is passed by reference. This is causing unintended side effects when the same solref object is used across multiple calls.

Reproduction

1. Pass the same solref list to multiple functions
2. Observe that modifications persist across calls
3. Confirmed with id(solref) remaining the same across calls

Current Behavior

farms_mujoco/farms_mujoco/simulation/mjcf.py

Line 256 in 8f02900

if solref is not None:

The function modifies the original list directly, causing subsequent calls to operate on already-modified values.

Expected Behavior

The function should operate on a copy of the list to prevent mutations of the original object.

Proposed Fix

Create a local copy of the list:

if solref is not None:
    scaled_solref = solref.copy()
    if all(sol < 0 for sol in local_solref):
        scaled_solref[0] *= units.newtons/units.meters
        scaled_solref[1] *= units.newtons/units.velocity
    else:
        scaled_solref[0] *= units.seconds

[jarreguit]

Good catch, please create a pull request when possible.