internal/mcp/protocol: move out of mcp/internal, as it's used in the API

The generated protocol package is part of the SDK API, so it shouldn't
be internal.

Change-Id: I5306af48019b1599707b939b348eb6ae4f766ca9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/670356
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/protocol/content.go

@@ -1,42 +0,0 @@
-// 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
-}