[2025-03-26 spec] Implement the Streamable HTTP transport

[mikekistler]

For compliance with the 2025-03-25 Model Context Protocol spec:

• Replace the previous HTTP+SSE transport with a more flexible Streamable HTTP transport (PR #206)

[halter73]

The stateful server-side part of this was added by #330.

[halter73]

Between #330 and #356, this has now been addressed. I still plan to add stateless Streamable HTTP support to the server that will allow you to use data protection to store information about the client like its capabilities, but I think that can be tracked seperately.

In the meantime, if you really want to write a stateless Streamable HTTP server today, you can see me do so using the JSON DTOs in this SDK and a plain minimal API here one YouTube. That's a timestamped link to that part of my demo in the ASP.NET Community Standup about Building MCP Servers with ASP.NET Core.