feat: multi-round-trip request implementation (SEP-2322) (#950)

### Context

Design (`design/mrtr.md`) and implementation proposal for Multi
Round-Trip Requests (MRTR) per
[SEP-2322](de6d76fba3/seps/2322-MRTR.md).

### Changes

- Introduce `InputRequest` and `InputResponse` sealed interfaces with
corresponding `InputRequestMap` and `InputResponseMap` types for custom
map JSON codec implementation.
- Extend `CallToolParams`, `GetPromptParams`, and `ReadResourceParams`
with `InputResponses`, `RequestState` and an unexported `resultType`
("complete" or "input_required") for retry round-trips. The type is set
by the SDK based on input requests presence.
- Add client-side MRTR middleware enabled by default that automatically
fulfills input requests and retries.
- Added MRTROptions on ClientOptions to allow disabling the middleware
and configuring the number of retries.
- Add server-side MRTR middleware for backward compatibility:
transparently bridges input requests to direct
Elicit/CreateMessage/ListRoots calls for older clients

mcp/client.go

@@ -58,13 +58,17 @@ func NewClient(impl *Implementation, options *ClientOptions) *Client {
 		opts.Logger = ensureLogger(nil)
 	}
 
-	return &Client{
+	c := &Client{
 		impl:                    impl,
 		opts:                    opts,
 		roots:                   newFeatureSet(func(r *Root) string { return r.URI }),
 		sendingMethodHandler_:   defaultSendingMethodHandler,
 		receivingMethodHandler_: defaultReceivingMethodHandler[*ClientSession],
 	}
+	if opts.MultiRoundTrip == nil || !opts.MultiRoundTrip.Disabled {
+		c.AddSendingMiddleware(clientMultiRoundTripMiddleware())
+	}
+	return c
 }
 
 // ClientOptions configures the behavior of the client.
@@ -154,6 +158,10 @@ type ClientOptions struct {
 	ResourceUpdatedHandler      func(context.Context, *ResourceUpdatedNotificationRequest)
 	LoggingMessageHandler       func(context.Context, *LoggingMessageRequest)
 	ProgressNotificationHandler func(context.Context, *ProgressNotificationClientRequest)
+	// MultiRoundTrip configures the automatic MultiRoundTrip (Multi Round-Trip Requests) middleware.
+	// By default (nil), the middleware is enabled with default settings.
+	// Set Disabled to true to opt out of automatic MultiRoundTrip handling.
+	MultiRoundTrip *MultiRoundTripOptions
 	// If non-zero, defines an interval for regular "ping" requests.
 	// If the peer fails to respond to pings originating from the keepalive check,
 	// the session is automatically closed.