An answer that fails to validate is asked for a second time, for that
classification only, with a reminder to reply with the JSON object alone.
The second ask comes out of the classification's CLASSIFICATION_DEADLINE:
it gets what the first ask left, and is skipped when less than 20 seconds
remain, so RUN_DEADLINE's bound still holds. A failed ask (runtime error,
timeout) is not repeated. If the second answer does not validate either,
the row has no answer, as before.
Model::ask takes the deadline for the ask; the ACP runtime no longer holds
its own.
The parser also takes the first fenced block of a reply that has prose
around the fence; prose around a bare object is still rejected.
ce7b3e055f