Support streamable HTTP

Streamable HTTP is the "new" way of doings things. HTTP + SSE events are the old way.

Once mcp-go supports this (https://github.com/mark3labs/mcp-go/issues/219), we should support it too.

EDIT: looks like it was added in https://github.com/mark3labs/mcp-go/pull/273, but i'm unsure how we would support it 🤔 