fix: constrained decoding cache bug and task rotation

[abrichr]

Summary

Fixes two bugs reported by customer on their first standalone trainer run:

1. Constrained decoding not activating: Outlines RegexLogitsProcessor failed silently on DFA compilation (the (.|\n)* prefix was too complex). Failure was cached as [] which passed the is not None check, so model.generate() received an empty logits_processor list — unconstrained output. Fixed: removed prefix from regex (model must output action directly), use False sentinel for failures.

2. Task rotation stuck on first task: _load_task_configs checked if not self._config.task_ids inside the loop — after appending the first task, the list was non-empty so remaining tasks were skipped. Fixed: check once before the loop.

Tests

21 new tests covering:

• Action regex: 8 valid actions match, 6 invalid texts rejected
• Cache sentinel: failure returns None (not []), success returns list
• Regression: empty list [] no longer treated as successful cache
• Task loading: all tasks from dir, explicit ids preserved, rotation math

All 21 pass in 0.03s, no GPU or server required.

Test plan

• 21/21 pytest tests pass
• Regex matches all valid action formats, rejects free-text
• Customer re-runs with constrained_decoding=True and verifies structured output

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