Add SSE keep-alive guidance to the Streamable HTTP transport doc

### Where was this agreed?

https://github.com/modelcontextprotocol/modelcontextprotocol/discussions/2905

### Description

Add a non-normative note to `basic/transports/streamable-http.mdx` recommending the SSE comment-line keep-alive (`:\r\n`) for long-lived `subscriptions/listen` streams, and stating that clients must not choke on SSE comment lines.