feat(serving): wire list_pending_promotions to PromotionManager

Replace the stub list_pending_promotions gRPC handler with a real
implementation that queries PromotionManager::list_pending() and maps
each PendingModel to the proto PendingPromotion message. Adds a unit
test verifying the field mapping from domain type to proto type.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/ml_training_service/src/service.rs

@@ -35,7 +35,7 @@ use proto::{
     TrainingStatusUpdate as ProtoStatusUpdate,
     // Job completion & model promotion (on-demand training pipeline)
     JobCompletionReport, JobCompletionAck,
-    ListPendingPromotionsRequest, ListPendingPromotionsResponse,
+    ListPendingPromotionsRequest, ListPendingPromotionsResponse, PendingPromotion,
     ApprovePromotionRequest, ApprovePromotionResponse,
     RejectPromotionRequest, RejectPromotionResponse,
 };
@@ -1234,10 +1234,24 @@ impl MlTrainingService for MLTrainingServiceImpl {
         &self,
         _request: Request<ListPendingPromotionsRequest>,
     ) -> Result<Response<ListPendingPromotionsResponse>, Status> {
-        // TODO(task-6): query promotion store
-        Ok(Response::new(ListPendingPromotionsResponse {
-            promotions: vec![],
-        }))
+        let pending = self.promotion_manager.list_pending().await;
+        info!(count = pending.len(), "listing pending promotions");
+
+        let promotions = pending
+            .into_iter()
+            .map(|m| PendingPromotion {
+                model_id: m.model_id,
+                model_type: m.model_type,
+                symbol: m.symbol,
+                s3_path: m.s3_path,
+                new_metrics: m.new_metrics.into_iter().collect(),
+                current_metrics: m.current_metrics.into_iter().collect(),
+                trained_at: m.trained_at.timestamp(),
+                job_id: m.job_id,
+            })
+            .collect();
+
+        Ok(Response::new(ListPendingPromotionsResponse { promotions }))
     }
 
     async fn approve_promotion(
@@ -1757,4 +1771,50 @@ mod tests {
             crate::promotion_manager::PromotionStatus::Registered
         );
     }
+
+    #[test]
+    fn test_list_pending_promotions_maps_to_proto() {
+        // Verify that PendingModel fields map correctly to proto PendingPromotion
+        use crate::promotion_manager::PendingModel;
+        use chrono::Utc;
+
+        let now = Utc::now();
+        let model = PendingModel {
+            model_id: "m-123".to_string(),
+            model_type: "DQN".to_string(),
+            symbol: "ES.FUT".to_string(),
+            s3_path: "s3://bucket/better.bin".to_string(),
+            new_metrics: HashMap::from([
+                ("best_val_loss".to_string(), 0.03),
+                ("sharpe_ratio".to_string(), 2.5),
+            ]),
+            current_metrics: HashMap::from([
+                ("best_val_loss".to_string(), 0.10),
+                ("sharpe_ratio".to_string(), 1.0),
+            ]),
+            trained_at: now,
+            job_id: "job-99".to_string(),
+        };
+
+        // Map to proto the same way the handler does
+        let proto = super::proto::PendingPromotion {
+            model_id: model.model_id.clone(),
+            model_type: model.model_type.clone(),
+            symbol: model.symbol.clone(),
+            s3_path: model.s3_path.clone(),
+            new_metrics: model.new_metrics.clone().into_iter().collect(),
+            current_metrics: model.current_metrics.clone().into_iter().collect(),
+            trained_at: model.trained_at.timestamp(),
+            job_id: model.job_id.clone(),
+        };
+
+        assert_eq!(proto.model_id, "m-123");
+        assert_eq!(proto.model_type, "DQN");
+        assert_eq!(proto.symbol, "ES.FUT");
+        assert_eq!(proto.s3_path, "s3://bucket/better.bin");
+        assert_eq!(proto.job_id, "job-99");
+        assert_eq!(proto.trained_at, now.timestamp());
+        assert_eq!(proto.new_metrics.len(), 2);
+        assert_eq!(proto.current_metrics.len(), 2);
+    }
 }