[MuJoCo] factorize _get_rew() out of step()

[Kallinteris-Andreas]

Description

factorizes the "reward function" of the step() function into its own function _get_rew(), similarly to _get_obs()

a change inspired by robosuite

motivation

makes the code easier to read/maintain, also should make port to MJX easier

validation

in the repo: https://github.com/Kallinteris-Andreas/gymnasium-changes-validation/tree/main/get_rew_factorization
I validated that over 1 mil steps the behavior is identical for all environments
and observed no measurable runtime slowdown

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Screenshots

Please attach before and after screenshots of the change if applicable.

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes