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/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, "")
 				}