Support SPN + RLS

Fabric Data Agent supports service principal (SPN) authentication, enabling agent-to-agent scenarios such as Microsoft Foundry calling a Data Agent using an SPN. However, SPN-based access is not currently supported with Row-Level Security (RLS). Supporting SPN + RLS would allow the Data Agent to preserve the appropriate user-level data permissions while using an SPN for service-to-service authentication, enabling more secure and flexible enterprise scenarios where both application identity and user identity need to be respected.