.NET: [PREVIEW BREAKING] Promote AgentSessionStore into Agents.AI.Abstractions - #7991

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: azure/login@a457da9ea143d694b1b9c7c869ebb04ebe844ef5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/