fix: retry unsuccessful idempotent subject turns

[neubig]

Actual Behavior

Run uv run pytest -q tests/test_subject_turns.py after reproducing a failed, canceled, or skipped subject attempt and resubmitting its original idempotency key. The later submission is deduplicated against the unsuccessful run, so the external issue or pull request cannot make progress. A timed-out subject behaves the same way after its runtime is released.

Desired Behavior

A stable subject submission should remain idempotent while work is active or successful, but it should be retryable after an unsuccessful attempt no longer owns a runtime.

Acceptance Criteria

• Repeated submissions remain deduplicated while the original subject run is queued, running, completed successfully, or still owns an unreleased runtime.
• The same idempotency key creates a new attempt after a failed, canceled, or skipped run has released its runtime.
• A run skipped before dispatch is treated as having no runtime and can be retried.
• The retry uses the same deterministic subject conversation identity.
• The unique idempotency record points to the current attempt while superseded runs remain in run history.
• Tests cover failed, canceled, skipped-before-start, and successful or active deduplication behavior.

[github-actions]

✅ ready-for-dev label applied

This issue meets the readiness criteria and is ready for contribution.
A contributor can now open a PR that links this issue (Fixes #<number>).

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