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