feat: partial reward support

[dzorlu]

Summary

• Adds partial_reward flag to FleetTaskEnv (off by default)
• When enabled, failed verifier runs compute a fractional score from ERROR_ACCUMULATOR / SUCCESS_ACCUMULATOR stdout instead of binary 0/1
• Passing tasks (score=1.0) are never modified
• Encapsulated in a _parse_partial_reward static method on FleetTaskEnv

Test plan

• Verify existing binary reward behavior unchanged when partial_reward=False
• Enable flag and verify partial scores appear in logs for failing tasks

🤖 Generated with Claude Code

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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Missing accumulator gives failed verifier a perfect score

High Severity

The guard not err_match and not suc_match only returns None when both accumulators are missing. When only SUCCESS_ACCUMULATOR is present (e.g., verifier crashed before printing ERROR_ACCUMULATOR), n_errors defaults to 0, so the partial score computes to n_success / n_success = 1.0. This silently overrides a genuinely failed verifier with a perfect reward of 1.0, corrupting the training signal. The condition likely needs to be not err_match or not suc_match to require both accumulators before computing a partial score.