fix(worker): 固定异步准入候选避免执行槽空转

异步任务在调度器准入后，Worker 会再次按实时负载排序候选；排序变化会让已准入任务退回 waiting，同时继续占用 River 执行槽，导致高并发吞吐塌陷。

执行前复用已持久化的 admitted 候选并稳定置顶，候选失效时仍保留原有重选路径；增加候选固定与非准入场景单测。

验证：Go 全量测试、runner race、go vet、OpenAPI 生成一致性、迁移安全检查通过。

apps/api/internal/runner/admission.go

@@ -187,6 +187,45 @@ func acceptanceAdmissionScopes(task store.GatewayTask, scopes []store.AdmissionS
 	return out
 }
 
+func (s *Service) loadAsyncTaskAdmission(ctx context.Context, task store.GatewayTask) (*store.TaskAdmission, error) {
+	if !task.AsyncMode {
+		return nil, nil
+	}
+	admission, err := s.store.GetTaskAdmission(ctx, task.ID)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &admission, nil
+}
+
+func pinCandidatesToTaskAdmission(
+	candidates []store.RuntimeModelCandidate,
+	admission *store.TaskAdmission,
+) ([]store.RuntimeModelCandidate, bool) {
+	if admission == nil || admission.Status != "admitted" || len(candidates) < 2 {
+		return candidates, false
+	}
+	pinnedIndex := -1
+	for index := range candidates {
+		if candidates[index].PlatformID == admission.PlatformID &&
+			candidates[index].PlatformModelID == admission.PlatformModelID {
+			pinnedIndex = index
+			break
+		}
+	}
+	if pinnedIndex <= 0 {
+		return candidates, false
+	}
+	out := append([]store.RuntimeModelCandidate(nil), candidates...)
+	pinned := out[pinnedIndex]
+	copy(out[1:pinnedIndex+1], out[:pinnedIndex])
+	out[0] = pinned
+	return out, true
+}
+
 func (s *Service) tryTaskAdmission(ctx context.Context, task store.GatewayTask, plan taskAdmissionPlan, waiterID string) (store.TaskAdmissionResult, error) {
 	return s.tryTaskAdmissionWithAdmittedHook(ctx, task, plan, waiterID, nil)
 }
@@ -195,17 +234,11 @@ func (s *Service) activeAsyncTaskAdmission(
 	ctx context.Context,
 	task store.GatewayTask,
 	plan taskAdmissionPlan,
+	admission *store.TaskAdmission,
 ) (store.TaskAdmissionResult, bool, error) {
-	if !task.AsyncMode {
+	if !task.AsyncMode || admission == nil {
 		return store.TaskAdmissionResult{}, false, nil
 	}
-	admission, err := s.store.GetTaskAdmission(ctx, task.ID)
-	if errors.Is(err, pgx.ErrNoRows) {
-		return store.TaskAdmissionResult{}, false, nil
-	}
-	if err != nil {
-		return store.TaskAdmissionResult{}, false, err
-	}
 	if admission.Status != "admitted" ||
 		admission.PlatformID != plan.Candidate.PlatformID ||
 		admission.PlatformModelID != plan.Candidate.PlatformModelID ||
@@ -220,7 +253,7 @@ func (s *Service) activeAsyncTaskAdmission(
 		return store.TaskAdmissionResult{}, false, nil
 	}
 	return store.TaskAdmissionResult{
-		Admission: admission,
+		Admission: *admission,
 		Admitted:  true,
 		Leases:    leases,
 	}, true, nil

apps/api/internal/runner/admission_test.go

@@ -61,3 +61,57 @@ func TestAcceptanceAdmissionScopesLeaveProductionPolicyUnchanged(t *testing.T) {
 		t.Fatalf("production admission policy changed: %+v", got[0])
 	}
 }
+
+func TestPinCandidatesToTaskAdmissionPreservesAdmittedCandidate(t *testing.T) {
+	input := []store.RuntimeModelCandidate{
+		{PlatformID: "platform-a", PlatformModelID: "model-a"},
+		{PlatformID: "platform-b", PlatformModelID: "model-b"},
+		{PlatformID: "platform-c", PlatformModelID: "model-c"},
+	}
+	admission := &store.TaskAdmission{
+		Status:          "admitted",
+		PlatformID:      "platform-b",
+		PlatformModelID: "model-b",
+	}
+
+	got, pinned := pinCandidatesToTaskAdmission(input, admission)
+
+	if !pinned {
+		t.Fatal("expected admitted candidate to be pinned")
+	}
+	if got[0].PlatformModelID != "model-b" || got[1].PlatformModelID != "model-a" || got[2].PlatformModelID != "model-c" {
+		t.Fatalf("unexpected candidate order: %+v", got)
+	}
+	if input[0].PlatformModelID != "model-a" {
+		t.Fatalf("candidate pinning mutated the caller slice: %+v", input)
+	}
+}
+
+func TestPinCandidatesToTaskAdmissionIgnoresWaitingOrMissingCandidate(t *testing.T) {
+	input := []store.RuntimeModelCandidate{
+		{PlatformID: "platform-a", PlatformModelID: "model-a"},
+		{PlatformID: "platform-b", PlatformModelID: "model-b"},
+	}
+	for name, admission := range map[string]*store.TaskAdmission{
+		"waiting": {
+			Status:          "waiting",
+			PlatformID:      "platform-b",
+			PlatformModelID: "model-b",
+		},
+		"missing": {
+			Status:          "admitted",
+			PlatformID:      "platform-c",
+			PlatformModelID: "model-c",
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			got, pinned := pinCandidatesToTaskAdmission(input, admission)
+			if pinned {
+				t.Fatalf("unexpected candidate pin for %s: %+v", name, got)
+			}
+			if got[0].PlatformModelID != "model-a" {
+				t.Fatalf("candidate order changed for %s: %+v", name, got)
+			}
+		})
+	}
+}

apps/api/internal/runner/service.go

@@ -422,6 +422,18 @@ func (s *Service) executeWithToken(ctx context.Context, task store.GatewayTask,
 			return Result{Task: failed, Output: failed.Result}, err
 		}
 	}
+	var asyncAdmission *store.TaskAdmission
+	if distributedAdmission && task.AsyncMode {
+		asyncAdmission, err = s.loadAsyncTaskAdmission(ctx, task)
+		if err != nil {
+			return Result{}, err
+		}
+		var pinned bool
+		candidates, pinned = pinCandidatesToTaskAdmission(candidates, asyncAdmission)
+		if pinned {
+			s.observeTaskAdmission("candidate_pinned")
+		}
+	}
 	pricingByCandidate := map[string]resolvedPricing{}
 	preprocessingByCandidate := map[string]parameterPreprocessResult{}
 	reservationBillings := []any(nil)
@@ -561,7 +573,7 @@ func (s *Service) executeWithToken(ctx context.Context, task store.GatewayTask,
 			var admissionErr error
 			if task.AsyncMode {
 				var alreadyAdmitted bool
-				admissionResult, alreadyAdmitted, admissionErr = s.activeAsyncTaskAdmission(ctx, task, plan)
+				admissionResult, alreadyAdmitted, admissionErr = s.activeAsyncTaskAdmission(ctx, task, plan, asyncAdmission)
 				if admissionErr == nil && !alreadyAdmitted {
 					admissionResult, admissionErr = s.tryTaskAdmission(ctx, task, plan, "")
 				}