internal/mcp: clean up handling of content

Add support for all content types, and formalize the conversion of
content to and from the wire format (as a discriminated union).

Change-Id: I93678ce98c02176a524d2c82425fb1267ad68bf0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

mcp/content_test.go

@@ -0,0 +1,75 @@
+// 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 (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+	"github.com/modelcontextprotocol/go-sdk/internal/protocol"
+)
+
+func TestContent(t *testing.T) {
+	tests := []struct {
+		in   mcp.Content
+		want protocol.Content
+	}{
+		{mcp.TextContent{Text: "hello"}, protocol.Content{Type: "text", Text: "hello"}},
+		{
+			mcp.ImageContent{Data: "a1b2c3", MimeType: "image/png"},
+			protocol.Content{Type: "image", Data: "a1b2c3", MIMEType: "image/png"},
+		},
+		{
+			mcp.AudioContent{Data: "a1b2c3", MimeType: "audio/wav"},
+			protocol.Content{Type: "audio", Data: "a1b2c3", MIMEType: "audio/wav"},
+		},
+		{
+			mcp.ResourceContent{
+				Resource: mcp.TextResource{
+					URI:      "file://foo",
+					MimeType: "text",
+					Text:     "abc",
+				},
+			},
+			protocol.Content{
+				Type: "resource",
+				Resource: &protocol.Resource{
+					URI:      "file://foo",
+					MIMEType: "text",
+					Text:     "abc",
+				},
+			},
+		},
+		{
+			mcp.ResourceContent{
+				Resource: mcp.BlobResource{
+					URI:      "file://foo",
+					MimeType: "text",
+					Blob:     "a1b2c3",
+				},
+			},
+			protocol.Content{
+				Type: "resource",
+				Resource: &protocol.Resource{
+					URI:      "file://foo",
+					MIMEType: "text",
+					Blob:     ptr("a1b2c3"),
+				},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		got := test.in.ToWire()
+		if diff := cmp.Diff(test.want, got); diff != "" {
+			t.Errorf("ToWire mismatch (-want +got):\n%s", diff)
+		}
+	}
+}
+
+func ptr[T any](t T) *T {
+	return &t
+}