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>

internal/protocol/content.go

@@ -0,0 +1,42 @@
+// 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 protocol
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// Content is the wire format for content, including all fields.
+type Content struct {
+	Type     string    `json:"type"`
+	Text     string    `json:"text,omitempty"`
+	MIMEType string    `json:"mimeType,omitempty"`
+	Data     string    `json:"data,omitempty"`
+	Resource *Resource `json:"resource,omitempty"`
+}
+
+// Resource is the wire format for embedded resources, including all fields.
+type Resource struct {
+	URI      string  `json:"uri,"`
+	MIMEType string  `json:"mimeType,omitempty"`
+	Text     string  `json:"text"`
+	Blob     *string `json:"blob"` // blob is a pointer to distinguish empty from missing data
+}
+
+func (c *Content) UnmarshalJSON(data []byte) error {
+	type wireContent Content // for naive unmarshaling
+	var c2 wireContent
+	if err := json.Unmarshal(data, &c2); err != nil {
+		return err
+	}
+	switch c2.Type {
+	case "text", "image", "audio", "resource":
+	default:
+		return fmt.Errorf("unrecognized content type %s", c.Type)
+	}
+	*c = Content(c2)
+	return nil
+}