cancel outstanding tasks if one task raises in generate

[rasdani]

Description

this fixes orphaned tasks sitting around, when a single task in generate raises an exception.

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

Prevents orphaned async tasks during rollout generation.

• In Environment.generate, wrap awaiting of as_completed coroutines with try/except; on any task exception, cancel all remaining tasks and drain them via asyncio.gather(..., return_exceptions=True) before re-raising
• No API changes; behavior only affects error handling and task cleanup

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