Avoid redundant recorder observation computation

[AntoineRichard]

Summary

• avoid the extra observation-manager compute when active recorder terms do not consume post-step observations
• preserve fresh post-step observations for existing custom recorder terms by default
• mark the built-in state and processed-action recorders as not requiring observations

This keeps RecorderManager.active_terms unchanged and limits the implementation to a recorder-term capability queried by ManagerBasedRLEnv.step().

Type of change

• Bug fix (non-breaking change which fixes an issue)

Release backport

• Backport this pull request to the active release branch after it merges into develop

Screenshots

Not applicable.

Validation

• uv run isaaclab -f
• uv run --extra test python -m pytest source/isaaclab/test/envs/test_manager_based_rl_env_unit.py source/isaaclab/test/managers/test_recorder_manager.py -q (9 passed)
• uv run python tools/changelog/cli.py check develop

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks
• Documentation is unchanged; a changelog fragment is included
• My changes generate no new warnings
• I have added tests that prove the fix is effective
• I have added a changelog fragment for the touched package
• My name is already present in CONTRIBUTORS.md