internal/mcp: meta and progress token

Meta is a struct that contains a map[string]any, and a ProgressToken
field.

Each Params and Result type has a Meta as a value field.

Meta marshals and unmarshals specially, so the ProgressToken field
becomes part of the map. The marshalling code is generalized in
anticipation of every Params and Result growing a map of additional
properties.

Change-Id: I6d2f34270d33501c96437f24f91ef3e51d20d955
Reviewed-on: https://go-review.googlesource.com/c/tools/+/674956
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

mcp/util_test.go

@@ -0,0 +1,48 @@
+// 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
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+)
+
+func TestMarshalStructWithMap(t *testing.T) {
+	type S struct {
+		A int
+		B string `json:"b,omitempty"`
+		u bool
+		M map[string]any `json:",omitempty"`
+	}
+	t.Run("basic", func(t *testing.T) {
+		s := S{A: 1, B: "two", M: map[string]any{"!@#": true}}
+		got, err := marshalStructWithMap(&s, "M")
+		if err != nil {
+			t.Fatal(err)
+		}
+		want := `{"A":1,"b":"two","!@#":true}`
+		if g := string(got); g != want {
+			t.Errorf("\ngot  %s\nwant %s", g, want)
+		}
+
+		var un S
+		if err := unmarshalStructWithMap(got, &un, "M"); err != nil {
+			t.Fatal(err)
+		}
+		if diff := cmp.Diff(s, un, cmpopts.IgnoreUnexported(S{})); diff != "" {
+			t.Errorf("mismatch (-want, +got):\n%s", diff)
+		}
+	})
+	t.Run("duplicate", func(t *testing.T) {
+		s := S{A: 1, B: "two", M: map[string]any{"b": "dup"}}
+		_, err := marshalStructWithMap(&s, "M")
+		if err == nil || !strings.Contains(err.Error(), "duplicate") {
+			t.Errorf("got %v, want error with 'duplicate'", err)
+		}
+	})
+}