fix(server): 显式 target_hosts 时主机指向用户指定的主节点

- 显式列表校验通过后令 host = nodes[0],task.TargetHost/IP、台账、
  CloudDM 注册与凭据地址不再指向此前默认挑选的空闲主机
- 显式列表解析抽为纯函数 resolvePinnedPrimaryReplicaNodes,
  行为与原内联逻辑一致(校验失败终态/单机上限 defer 重试)
- 新增测试覆盖主节点非池首空闲节点及数量/池外/重复/占满场景

server/internal/service/delivery.go

@@ -335,6 +335,40 @@ func hostTaskCount(occupied []string, name string) int {
 	return count
 }
 
+// resolvePinnedPrimaryReplicaNodes 按用户显式指定的有序 target_hosts 解析 primary_replica 节点列表，
+// 首节点为主节点。返回值：节点列表、参数校验失败原因（终态）、延后调度错误（defer: 前缀，可重试）。
+func resolvePinnedPrimaryReplicaNodes(hosts []targetHost, requested []string, occupied []string, limit int, nodeCount int64, pinnedPrimary string) ([]*targetHost, string, error) {
+	if int64(len(requested)) != nodeCount {
+		return nil, fmt.Sprintf("primary_replica requires %d target_hosts, got %d", nodeCount, len(requested)), nil
+	}
+	nodes := make([]*targetHost, 0, len(requested))
+	seen := map[string]struct{}{}
+	for index, requestedHost := range requested {
+		var selected *targetHost
+		for i := range hosts {
+			if hosts[i].Name == requestedHost {
+				selected = &hosts[i]
+				break
+			}
+		}
+		if selected == nil {
+			return nil, fmt.Sprintf("target_hosts[%d] %q is not in the candidate host pool", index, requestedHost), nil
+		}
+		if _, exists := seen[selected.Name]; exists {
+			return nil, fmt.Sprintf("target_hosts contains duplicate host %q", selected.Name), nil
+		}
+		if hostTaskCount(occupied, selected.Name) >= max(limit, 1) {
+			return nil, "", fmt.Errorf("defer: pinned host %s reached the per-host instance limit %d", selected.Name, max(limit, 1))
+		}
+		seen[selected.Name] = struct{}{}
+		nodes = append(nodes, selected)
+	}
+	if pinnedPrimary != "" && nodes[0].Name != pinnedPrimary {
+		return nil, "target_host must match the first primary_replica target_hosts entry", nil
+	}
+	return nodes, "", nil
+}
+
 func topologyNodeCount(input MySQLDeliveryInput) int64 {
 	if input.Topology == "primary_replica" {
 		replicas := input.ReplicaCount
@@ -1757,34 +1791,17 @@ func (s *DeliveryService) claimAndReserve(ctx context.Context) (*model.DeliveryT
 		nodes := []*targetHost{host}
 		if nodeCount > 1 {
 			if len(payload.TargetHosts) > 0 {
-				if int64(len(payload.TargetHosts)) != nodeCount {
-					return s.failInTransaction(tx, &task, model.TaskValidationFailed, fmt.Sprintf("primary_replica requires %d target_hosts, got %d", nodeCount, len(payload.TargetHosts)))
+				pinned, validationMsg, deferErr := resolvePinnedPrimaryReplicaNodes(meta.Hosts, payload.TargetHosts, occupied, limit, nodeCount, payload.TargetHost)
+				if deferErr != nil {
+					return deferErr
 				}
-				nodes = nodes[:0]
-				seen := map[string]struct{}{}
-				for index, requestedHost := range payload.TargetHosts {
-					var selected *targetHost
-					for i := range meta.Hosts {
-						if meta.Hosts[i].Name == requestedHost {
-							selected = &meta.Hosts[i]
-							break
-						}
-					}
-					if selected == nil {
-						return s.failInTransaction(tx, &task, model.TaskValidationFailed, fmt.Sprintf("target_hosts[%d] %q is not in the candidate host pool", index, requestedHost))
-					}
-					if _, exists := seen[selected.Name]; exists {
-						return s.failInTransaction(tx, &task, model.TaskValidationFailed, fmt.Sprintf("target_hosts contains duplicate host %q", selected.Name))
-					}
-					if hostTaskCount(occupied, selected.Name) >= max(limit, 1) {
-						return fmt.Errorf("defer: pinned host %s reached the per-host instance limit %d", selected.Name, max(limit, 1))
-					}
-					seen[selected.Name] = struct{}{}
-					nodes = append(nodes, selected)
-				}
-				if payload.TargetHost != "" && nodes[0].Name != payload.TargetHost {
-					return s.failInTransaction(tx, &task, model.TaskValidationFailed, "target_host must match the first primary_replica target_hosts entry")
+				if validationMsg != "" {
+					return s.failInTransaction(tx, &task, model.TaskValidationFailed, validationMsg)
 				}
+				// 显式列表以首节点为主节点：覆盖此前默认挑选的空闲主机，
+				// 避免 task.TargetHost/IP 与台账、凭据指向未部署节点。
+				nodes = pinned
+				host = nodes[0]
 			} else {
 				for i := range meta.Hosts {
 					candidate := &meta.Hosts[i]