learn: score_ranges is now required; short-circuit on empty

vllm's /v1/score endpoint made score_ranges a required field (the
messages-mode fallback that used to pattern-scan for assistant
boundaries is gone). Always send the field, and if we have nothing to
score, skip the HTTP round-trip entirely instead of letting the server
422 us.

Response parsing is unchanged — serde ignores the renamed range_index
field and the dropped role field since we only extract total_logprob.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/subconscious/learn.rs

@@ -129,16 +129,18 @@ async fn call_score(
     ranges: &[(usize, usize)],
     priority: Option<i32>,
 ) -> anyhow::Result<Vec<ScoreResult>> {
+    // Nothing to score — skip the round-trip.
+    if ranges.is_empty() {
+        return Ok(Vec::new());
+    }
     let url = format!("{}/score", client.base_url());
     let auth = format!("Bearer {}", client.api_key());
     let mut body = serde_json::json!({
         "model": client.model,
         "prompt": prompt,
+        "score_ranges": ranges,
         "logprobs": 1,
     });
-    if !ranges.is_empty() {
-        body["score_ranges"] = serde_json::json!(ranges);
-    }
     if let Some(p) = priority {
         body["priority"] = serde_json::json!(p);
     }