internal/mcp: an MCP SDK prototype

This CL contains an minimal prototype of an MCP SDK, using the
"new" jsonrpc2_v2 package (similar to x/exp/jsonrpc2).

Much is still yet to do, but this initial version addressed the
following aspects:

- Support for newline delimited JSONRPC2 framing, and message logging.
- A generated protocol package, containing type definitions to be used
  in transport.
- A minimal jsonschema package to be used for both reading the MCP spec,
  and for serving Tool input schemas.
- A transport abstraction, and two transport implementations: stdio, and
  local (for testing).
- Client and Server types, to be configured with features and then
  connected using their Connect methods, which return (respectively) a
  ServerConnection and ClientConnection object.
- A minimal binding API for tools.

A catalog of things not yet done is in doc.go, but we should review this
CL eagerly, as it contains the fundamental building blocks of an
eventual SDK.

It is the intention that this package is more-or-less standalone, as it
may eventually be carved out into a separate repository. As such, its
only dependency within x/tools is jsonrpc2_v2, and the only change
required to that package is to expose a callback when the connection is
closed.

Change-Id: Ib66018961014f85a8884d56ab721d5642c98443c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/667036
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

examples/hello/main.go

@@ -0,0 +1,38 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+type Optional[T any] struct {
+	present bool
+	value   T
+}
+
+type SayHiParams struct {
+	Name string `json:"name" mcp:"the name to say hi to"`
+}
+
+func SayHi(ctx context.Context, params *SayHiParams) ([]mcp.Content, error) {
+	return []mcp.Content{
+		mcp.TextContent{Text: "Hi " + params.Name},
+	}, nil
+}
+
+func main() {
+	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)
+	}
+}