Expose ranking metadata in request trace

apps/aether-gateway/src/handlers/admin/observability/monitoring/tests/trace.rs

@@ -358,6 +358,66 @@ async fn admin_monitoring_trace_request_enriches_proxy_timing_from_usage_audit()
     );
 }
 
+#[tokio::test]
+async fn admin_monitoring_trace_request_exposes_structured_ranking_metadata() {
+    let mut candidate = sample_candidate(
+        "cand-used",
+        "request-1",
+        0,
+        RequestCandidateStatus::Success,
+        Some(101),
+        Some(33),
+        Some(200),
+    );
+    candidate.extra_data = Some(json!({
+        "ranking_mode": "CacheAffinity",
+        "priority_mode": "Provider",
+        "ranking_index": 0,
+        "priority_slot": 7,
+        "promoted_by": "cached_affinity",
+        "demoted_by": "cross_format",
+        "client_api_format": "openai:responses"
+    }));
+
+    let request_candidates = Arc::new(InMemoryRequestCandidateRepository::seed(vec![candidate]));
+    let provider_catalog = Arc::new(InMemoryProviderCatalogReadRepository::seed(
+        vec![sample_provider()],
+        vec![sample_endpoint()],
+        vec![sample_key()],
+    ));
+    let state = AppState::new()
+        .expect("state should build")
+        .with_decision_trace_data_readers_for_tests(request_candidates, provider_catalog);
+    let context = request_context(http::Method::GET, "/api/admin/monitoring/trace/request-1");
+
+    let response = local_monitoring_response(&state, &context)
+        .await
+        .expect("handler should not error")
+        .expect("route should be handled locally");
+
+    assert_eq!(response.status(), http::StatusCode::OK);
+    let body = to_bytes(response.into_body(), usize::MAX)
+        .await
+        .expect("body should read");
+    let payload: serde_json::Value = serde_json::from_slice(&body).expect("json body should parse");
+
+    assert_eq!(
+        payload["candidates"][0]["ranking"],
+        json!({
+            "mode": "CacheAffinity",
+            "priority_mode": "Provider",
+            "index": 0,
+            "priority_slot": 7,
+            "promoted_by": "cached_affinity",
+            "demoted_by": "cross_format"
+        })
+    );
+    assert_eq!(
+        payload["candidates"][0]["extra_data"]["ranking_mode"],
+        json!("CacheAffinity")
+    );
+}
+
 #[tokio::test]
 async fn admin_monitoring_trace_provider_stats_returns_local_payload() {
     let request_candidates = Arc::new(InMemoryRequestCandidateRepository::seed(vec![