mcp: support SSE polling and server disconnects (SEP-1699) (#663)

This CL implements support for SEP-1699, adding the following new API:

- Event.Retry corresponds to the 'retry' field of a server sent event.
This is used to signal client retry behavior.
- RequestExtra.CloseStream is set by the streamable transport to allow
closing a stream.

The streamable server transport is updated to set CloseStream for calls,
allowing server operations or middleware to close the underlying stream.
If the configured 'reconnectAfter' delay is set, a 'retry' message is
sent prior to terminating the stream.

The streamable client transport is updated to read the configured delay.

Fixes #630

mcp/shared.go

@@ -468,6 +468,14 @@ type ServerRequest[P Params] struct {
 type RequestExtra struct {
 	TokenInfo *auth.TokenInfo // bearer token info (e.g. from OAuth) if any
 	Header    http.Header     // header from HTTP request, if any
+
+	// CloseStream closes the current request stream, if the current transport
+	// supports replaying requests.
+	//
+	// If reconnectAfter is nonzero, it signals to the client to reconnect after
+	// the given duration. Otherwise, clients may determine their own
+	// reconnection policy.
+	CloseStream func(reconnectAfter time.Duration)
 }
 
 func (*ClientRequest[P]) isRequest() {}