An agent stage that failed billed nothing: the backend returned a bare
error and the outcome built from it carried no usage. A terminal failure
now becomes the stage's failed outcome from the same fold that bills a
completed stage, with the tree's usage, the rows by model, the files it
wrote, and its active time; stage.failed carries billing and
billing_by_model and the store keeps both. Cancellation and retryable
failures still go up as the error.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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