Allow HybridCache for session storage

[delepster]

Is your feature request related to a problem? Please describe.
In a scenario where multiple instances of an MCP server are horizontally scaled and load-balanced (e.g. k8s), the session ID needs to be shared between the instances

Describe the solution you'd like
The StatefulSessionManager class is sealed, and there is no interface implementation available so that we could use a distributed cache like HybridCache to allow for session sharing. Either:

• create an interface to allow custom session management; or
• use HybridCache to store sessions in-memory while allowing for an external secondary cache (e.g. Redis)

Describe alternatives you've considered

• Overriding the StatefulSessionManager class
• Update the infrastructure to leverage sticky by a proxy

[geffzhang]

The current C# MCP SDK's HttpStream connection relies on session affinity and does not support Backplane. We need to learn from SignalR's architectural patterns and introduce a distributed backplane similar to ASP.NET Core SignalR's.

[delepster]

Wouldn't it be acceptable to just use the HybridCache in StatefulSessionManager instead of a ConcurrentDictionary ?