internal/mcp: add a command transport for connecting to a sidecar

Add a missing CommandTransport that communicates with a sidecar process
over newline-delimited stdin/stdout, the counterpart to the existing
StdIOTransport.

Add a test using the usual TestMain trick.

Change-Id: I170ba78faf6b6d7fca7da1af11bf92e919b11ed5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

mcp/cmd_test.go

@@ -0,0 +1,67 @@
+// 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 mcp_test
+
+import (
+	"context"
+	"log"
+	"os"
+	"os/exec"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+const runAsServer = "_MCP_RUN_AS_SERVER"
+
+func TestMain(m *testing.M) {
+	if os.Getenv(runAsServer) != "" {
+		os.Unsetenv(runAsServer)
+		runServer()
+		return
+	}
+	os.Exit(m.Run())
+}
+
+func runServer() {
+	ctx := context.Background()
+
+	server := mcp.NewServer("greeter", "v0.0.1", nil)
+	server.AddTools(mcp.MakeTool("greet", "say hi", SayHi))
+
+	if err := server.Run(ctx, mcp.NewStdIOTransport(), nil); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func TestCmdTransport(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmd := exec.Command(exe)
+	cmd.Env = append(os.Environ(), runAsServer+"=true")
+
+	client := mcp.NewClient("client", "v0.0.1", nil)
+	serverConn, err := client.Connect(ctx, mcp.NewCommandTransport(cmd), nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+	got, err := serverConn.CallTool(ctx, "greet", SayHiParams{Name: "user"})
+	if err != nil {
+		log.Fatal(err)
+	}
+	want := []mcp.Content{mcp.TextContent{Text: "Hi user"}}
+	if diff := cmp.Diff(want, got); diff != "" {
+		t.Errorf("greet returned unexpected content (-want +got):\n%s", diff)
+	}
+	if err := serverConn.Close(); err != nil {
+		t.Fatalf("closing server: %v", err)
+	}
+}