Handle CP OPSM masks centrally and restore loss guardrails

[cklxx]

Summary

• refactor OPSM mask computation to be context-parallel aware without duplicating logic in the loss
• restore gradient connectivity when policy log-prob tensors are empty to avoid distributed hangs
• validate rollout buffer group sizes to prevent silently misaligned sample counts

Testing

• python -m ruff check slime slime_plugins tests
• python -m pytest (fails: missing optional dependencies such as flash_attn and model-specific test modules)

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Codex Task