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-review-classifier/src/main.rs

@@ -67,7 +67,6 @@ async fn serve() -> Result<()> {
         acp: acp::Acp {
             program: PathBuf::from(var("OPENCODE")?),
             trampoline,
-            deadline: swarm_queue_client::review_classifier::CLASSIFICATION_DEADLINE,
             cancel_wait: acp::CANCEL_WAIT,
             cost: cost::Otel,
             cwd: std::env::temp_dir().join("review-classifier-acp-cwd"),