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/prompt.rs

@@ -54,6 +54,10 @@ pub fn shared_context(run: &TryOut) -> String {
     out
 }
 
+/// Appended to the question when it is asked a second time.
+const JSON_REMINDER: &str = "Your previous answer was not a JSON object. Reply with the JSON \
+     object and nothing else: no text before or after it, and no code fence.";
+
 /// One classification's prompt: the run's shared context unchanged, then the
 /// question.
 pub struct Prompt<'a> {
@@ -67,6 +71,16 @@ impl Prompt<'_> {
     pub fn text(&self) -> String {
         format!("{}{}", self.shared, self.question)
     }
+
+    /// The same prompt with a reminder to answer with the JSON object alone,
+    /// for asking again after an answer that was not that object.
+    #[must_use]
+    pub fn with_json_reminder(&self) -> Self {
+        Self {
+            shared: self.shared,
+            question: format!("{}\n{JSON_REMINDER}\n", self.question.trim_end()),
+        }
+    }
 }
 
 #[cfg(test)]