fix(worker): 固定等待任务的准入候选

双 Worker 会按动态运行态排序重复规划同一批 waiting 任务，导致候选绑定来回迁移并把同步事务消耗在 rebind 上。\n\n调度 waiting 任务时优先使用首次登记的候选；仅当候选不再可用或已不在合法候选集时才按现有顺序迁移。执行阶段的 admitted 候选固定逻辑保持不变。\n\n验证：新增 waiting 候选固定回归测试；Go 全量测试、go vet、gofmt 通过。

apps/api/internal/runner/admission.go

@@ -66,6 +66,15 @@ func distributedAdmissionModelType(modelType string) bool {
 }
 
 func (s *Service) buildTaskAdmissionPlan(ctx context.Context, task store.GatewayTask, user *auth.User) (taskAdmissionPlan, error) {
+	return s.buildTaskAdmissionPlanForCurrentBinding(ctx, task, user, nil)
+}
+
+func (s *Service) buildTaskAdmissionPlanForCurrentBinding(
+	ctx context.Context,
+	task store.GatewayTask,
+	user *auth.User,
+	admission *store.TaskAdmission,
+) (taskAdmissionPlan, error) {
 	restoredRequest, err := s.restoreTaskRequestReferences(ctx, task)
 	if err != nil {
 		return taskAdmissionPlan{}, err
@@ -107,6 +116,7 @@ func (s *Service) buildTaskAdmissionPlan(ctx context.Context, task store.Gateway
 	if err != nil {
 		return taskAdmissionPlan{}, err
 	}
+	candidates, _ = pinCandidatesToTaskAdmission(candidates, admission)
 	for _, candidate := range candidates {
 		available, availabilityErr := s.store.RuntimeCandidateAvailable(ctx, candidate.PlatformID, candidate.PlatformModelID)
 		if availabilityErr != nil {
@@ -220,7 +230,9 @@ func pinCandidatesToTaskAdmission(
 	candidates []store.RuntimeModelCandidate,
 	admission *store.TaskAdmission,
 ) ([]store.RuntimeModelCandidate, bool) {
-	if admission == nil || admission.Status != "admitted" || len(candidates) < 2 {
+	if admission == nil ||
+		(admission.Status != "waiting" && admission.Status != "admitted") ||
+		len(candidates) < 2 {
 		return candidates, false
 	}
 	pinnedIndex := -1
@@ -629,7 +641,14 @@ func (s *Service) dispatchWaitingAsyncTasks(ctx context.Context, tasks []store.G
 	tasksByID := make(map[string]store.GatewayTask, len(tasks))
 	for _, task := range tasks {
 		user := authUserFromTask(task)
-		plan, err := s.buildTaskAdmissionPlan(ctx, task, user)
+		current, err := s.store.GetTaskAdmission(ctx, task.ID)
+		if err != nil {
+			if errors.Is(err, pgx.ErrNoRows) {
+				continue
+			}
+			return false, err
+		}
+		plan, err := s.buildTaskAdmissionPlanForCurrentBinding(ctx, task, user, &current)
 		if err != nil {
 			return false, err
 		}