SEP-1699: support SSE polling

[findleyr]

If SEP-1699 lands for the 2025-11-25 version of the spec, we must implement it.

(This one should fall to me).

[findleyr]

I propose the following new API to support this:

type RequestExtra struct {
  ...
  CloseStream func(reconnectAfter time.Duration) // Close the request stream, for streamable transports
}

This is actually in contrast to the pending change to the typescript SDK, which puts this on the transport (https://github.com/modelcontextprotocol/typescript-sdk/pull/1129/files).

We try not to leak information from the transport to the MCP layer, except in specifically prescribed ways, which are encapsulated by RequestExtra, which has documentation "extra information included in requests, typically from the transport layer." I think this is the best place for a CloseStream callback in our current model.

[findleyr]

Err, we may want both the API above, and a method on StreamableServerTransport. The latter could be useful in (for example) http middleware that closes streams after some time. I guess I need clarity on the intended usage.