Motivation & Context

Applications that switch the model or provider handling an existing conversation need to preserve the conversation history across that switch. Without shared client-side history, the newly selected provider loses the prior context and applications must rehydrate it manually.

Microsoft.Extensions.AI now provides the routing mechanism through its RoutingChatClient base type. Agent Framework still needs an agent-aware implementation that persists the active route in AgentSession state, allowing developers to inspect or change the route while keeping conversation history portable between routed clients.

Description & Review Guide

• What are the major changes?
  • Adds experimental RoutePersistingRoutingChatClient, derived from the MEAI RoutingChatClient, with mutable named routes and per-session active-route persistence.
  • Adds GetActiveRoute and SetActiveRoute, configurable default/state keys, optional inner-client ownership, lazy route validation, service forwarding, and AOT-safe state serialization.
  • Updates Microsoft.Extensions.AI dependencies needed for the MEAI routing API and related transitive package versions.
  • Adds 37 focused unit tests and a multi-model routing sample demonstrating route changes with client-side conversation history.
• What is the impact of these changes?
  • This is additive, experimental .NET API. Developers can add, replace, or remove routes after construction and switch a session between clients without manually rehydrating client-side chat history.
  • Routed clients must not use service-stored chat history, because that history cannot be shared across routes.
• What do you want reviewers to focus on?
  • The lazy validation semantics for mutable routes, persisted-session behavior when routes are removed or replaced, and ownership/disposal behavior.

Related Issue

Fixes #6865

This supersedes #6932: the generic routing mechanism from that PR was moved into Microsoft.Extensions.AI in dotnet/extensions#7662. This PR builds on the MEAI RoutingChatClient and adds only the Agent Framework-specific session persistence layer.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" in the title prefix, before or after the language prefix) — a workflow keeps the label and title prefix in sync automatically.