Multi-Label Support for Actions Runner Controller (ARC) [GA]

### Value Prop

Multi-label support in Actions Runner Controller (ARC) gives enterprises the flexibility to manage complex workflows without duplicating infrastructure. Instead of maintaining separate scale sets for each configuration, customers can assign multiple labels. For example a single scale set could have labels that represent attributes for operating system, hardware, compliance, and network configuration. This capability simplifies runner management, reduces operational overhead, and ensures jobs land on the right compute resources with minimal effort.

### Expected Outcome

Customers gain a streamlined approach to autoscaling and job routing, improving resource utilization and reducing idle capacity. This results in lower infrastructure costs, faster execution, and better alignment with compliance and performance requirements, delivering an efficient and scalable self-hosted runner experience.

该 issue 未提供文件、测试或代码入口；首先识别负责 scale-set 标签和作业路由的 Actions Runner Controller (ARC) 组件。明确实现范围和验收标准，将针对操作系统、硬件、compliance 和网络属性的多标签分配与正确路由定义为完成条件。

由索引模型根据 Issue 内容生成。