review-classifier: ask a check once more when its answer isn't valid JSON

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.

swarm-queue-client/src/review_classifier.rs

@@ -23,8 +23,8 @@ pub const QUEUE_GROUP: &str = "review-classifier";
 /// The most classifications one run carries.
 pub const MAX_CLASSIFICATIONS: usize = 10;
 
-/// The longest one classification's model run may take before it is stopped
-/// and its row carries a timeout.
+/// The longest one classification may take, its first ask and any second ask
+/// together, before it is stopped and its row carries a timeout.
 pub const CLASSIFICATION_DEADLINE: Duration = Duration::from_mins(3);
 
 /// The longest a whole run may take: every classification at its deadline,