internal/mcp: implement HTTP SSE transport

Add support for SSE transport, as defined by version 2024-11-05 and
2025-03-26 of the MCP spec.

In order to achieve this, the jsonrpc2 stream must be abstracted so that
we can implement it across multiple HTTP requests. This required some
refactoring of the jsonrpc2_v2 package, so that we can avoid the
Dialer/Binder interfaces and construct the Connection directly from its
constituent primitives.

Change-Id: Id627b5b5a474d92f2c613cfad904d14611e7c768
Reviewed-on: https://go-review.googlesource.com/c/tools/+/667475
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

examples/hello/main.go

@@ -6,16 +6,15 @@ package main
 
 import (
 	"context"
+	"flag"
 	"fmt"
+	"net/http"
 	"os"
 
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 )
 
-type Optional[T any] struct {
-	present bool
-	value   T
-}
+var httpAddr = flag.String("http", "", "if set, use SSE HTTP at this address, instead of stdin/stdout")
 
 type SayHiParams struct {
 	Name string `json:"name" mcp:"the name to say hi to"`
@@ -28,11 +27,20 @@ func SayHi(ctx context.Context, params *SayHiParams) ([]mcp.Content, error) {
 }
 
 func main() {
+	flag.Parse()
+
 	server := mcp.NewServer("greeter", "v0.0.1", nil)
 	server.AddTools(mcp.MakeTool("greet", "say hi", SayHi))
 
-	opts := &mcp.ConnectionOptions{Logger: os.Stderr}
-	if err := server.Run(context.Background(), mcp.NewStdIOTransport(), opts); err != nil {
-		fmt.Fprintf(os.Stderr, "Server failed: %v", err)
+	if *httpAddr != "" {
+		handler := mcp.NewSSEHandler(func() *mcp.Server {
+			return server
+		})
+		http.ListenAndServe(*httpAddr, handler)
+	} else {
+		opts := &mcp.ConnectionOptions{Logger: os.Stderr}
+		if err := server.Run(context.Background(), mcp.NewStdIOTransport(), opts); err != nil {
+			fmt.Fprintf(os.Stderr, "Server failed: %v", err)
+		}
 	}
 }