Expose render_completion + add_trajectory_step, private/final guardrails

[willccbb]

Description

Elevates render_completion to an overridable MultiTurnEnv method, and adds an overridable method add_trajectory_step for setting step-level rewards/advantages/extras. Logic not intended for overriding is marked as final if public-facing, private if internal. Additional methods in Environment/EnvGroup are marked as private/final (or removed if no longer needed).

Minor reorganizations + docstring improvements.

Type of Change

• 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)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• 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
• Any dependent changes have been merged and published

Additional Notes

---

Note

API additions and overrides

• MultiTurnEnv: new overridable render_completion and add_trajectory_step; rollout marked final; env_response is abstract; completion rendered at end of rollout.
• experimental/RLMEnv: implements render_completion to ignore sub-LLM steps when computing completion.

Core refactors and guardrails

• Environment: internal helpers renamed private (_format_dataset, _format_completion_dataset, _get_eval_inputs); several public methods marked final (get_dataset, get_eval_dataset, init_state, is_completed, run_rollout, run_group); removed internal completion rendering; minor doc/docstring updates.
• EnvGroup: aligns to private dataset formatters (_format_dataset, _format_completion_dataset); rollout marked final; docstrings clarified.

Misc

• ToolRubric and data_utils: minor typing/cast cleanups without behavior changes.

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