Support new MCP Streaming HTTP transport

[JAORMX]

The newest spec replaced the HTTP SSE transport with Streaming HTTP. It can be found here: https://spec.modelcontextprotocol.io/specification/2025-03-26/basic/transports/#streamable-http we should support it.

[ChrisJBurns]

We should maybe do this sooner rather than later, as not only is Streaming HTTP out, but SSE has been deprecated also.

[danbarr]

Piling on, in VS Code 1.100, SSE fallback is broken. Supposedly fixed in Insiders, but not for me.

microsoft/vscode#246753