Add stateless Streamable HTTP support

[halter73]

• This allows a single MCP session spanning multiple requests to be handled by different servers without sharing state
• This does require the servers share data protection keys, but this is standard for ASP.NET Core cookies and antiforgery as well

I don't think we have an issue tracking this precisely, but this has been previously discussed in #347 and #330 (comment)

[stephentoub]

No changes are necessary on the client?

[rrudduck]

Any ETA on this getting merged and a new package released? Happy to help test, I can replicate the issue this resolves pretty easily.

[halter73]

Any ETA on this getting merged and a new package released? Happy to help test

We're looking at getting out on NuGet early next week. Hopefully, Monday. I want to resolve some issues like sampling requests currently hanging on the server rather than producing a clear error that sampling is not supported in stateless mode, since there is no guarantee that the sampling response will come back to the same server process like we expect in the default stateful scenario.

If you want to test this before then, I welcome you cloning https://github.com/halter73/csharp-sdk/tree/stateless and trying it out yourself. Maybe you can provide feedback to improve it before we ship it on NuGet.

I can replicate the issue this resolves pretty easily.

What issue are you referring to specifically? I'm just curious. This resolves a couple issues like running behind a non-sticky load balancer and supporting indefinite sessions that outlive even a server restart. It's a tradeoff since it doesn't support some features like sampling and roots, but that's a tradeoff I think many MCP server developers will be willing to take.

[rrudduck]

What issue are you referring to specifically? I'm just curious. This resolves a couple issues like running behind a non-sticky load balancer and supporting indefinite sessions that outlive even a server restart. It's a tradeoff since it doesn't support some features like sampling and roots, but that's a tradeoff I think many MCP server developers will be willing to take.

Sorry, should have specified that in my original response. It's the scenario when running multiple replicas without affinity. Since these MCP servers live a number of layers down in our service stack it's difficult to affinitize requests all the way through. Until I saw your changes, we were looking at hooking in either some sort of intelligent router or a distributed session store (if we could plumb it in somehow). Your changes are a much simpler way to solve it.